(Whoa! This post turned out to be way long. Bear with me. . .

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I have a feeling she's more upset about not being able to renegotiate her compensation, after she's seen how important she apparently is to the show. That's the downside of contracts.
Yes, I've been thinking the same thing.

I've been puzzling about her Twitter statement since I read it.
It sounds like she was in the middle of a conversation, or responding to a bunch of people, so the comment doesn't have a proper context to understand where she's coming from. When I looked at her Twitter account, there was no other side of any conversations where this came out of.
This is my hypothesis of what has been going on:

I saw an interview with actor, Rob Lowe, recently. He talked about how he had walked away from the show,
The West Wing, when it was at the height of it's popularity. He said he helped make the show so successful. But, when contract negotiation time came about, all his co-stars got significant pay raises and he didn't.
He said, what we public do not realize is how large these TV studios are. That they are HUGE corporations. The executives at the top of the studios who make the ultimate hiring and firing and pay raise decisions are so far up the food chain & extremely far removed from the actual shows, producers and actors. It is just this big board room of strangers, who've never met the actors, who've probably never seen most of their own studio's TV shows. They look at numbers on charts or whatever, and they make these HUGE decisions which affect the actors who bring in all the viewers.
Rob said he tried to play along at first. But, not only did his salary not go up, his role started getting cut. They just did not appreciate his talent or contributions or showed they valued him as a member like his co-workers. Finally, for his own integrity, dignity and self-respect, he needed to walk away. He said he would encourage anyone, even his own sons, that if they ever get into such situations, to have the courage and self respect to walk away also.
I think what the head studio execs at ABC/CBS have done to
Criminal Minds, is like: One can be working for a great company where one really loves their co-workers and their job,

but the higher ups, who aren't even part of the day to day operations, but make decisions which affect the overall operations as a whole, have made it an untenable work situation.
I think Thomas Gibson (Hotch) is at the same point as Rob Lowe was. Especially after seeing what happened to J.J. and Prentiss. He may have to walk away if he isn't offered what his contributions are worth. The show is in the top 10 in the ratings for ALL shows, on ALL networks, but look at what these studio execs had done to the show last season for
financial reasons.

It's clear they don't watch it. They had no understanding how firing BOTH J.J. and Prentiss would affect the Team and the viewers. They thought we wouldn't notice or mind a cheaper faux J.J. brought in.
Yes, they brought the two actresses back, but they seem to still not get it. Now, it's about the guys. I have NEVER seen a show where two actors, (Hotch & Morgan) were renegotiating contracts and the writers actually had to write in story lines where the future of the two characters and the actors was unknown!!!
Look how long it took for Morgan to get a new contract. It is now going into
three weeks since his contract was renegotiated, and Thomas STILL does not have a new one.

If the rumors are true that Morgan got nearly double his salary, then Thomas deserves at least that, too.
When that original contract for Paget (Prentiss) was being re-negotiated last season, she had been fired from CM completely. She had been even fighting for how many episodes she'd have left. I think her agent was able to get her about 2 more episodes than the execs had originally planned. I remember when she had Twittered that this season was her
definitely her last year.
Below is an interview with J.J. who talks about how she never got real closure when she left the show as they kept calling her and asking her back to do more things. (She was fired completely in June. Then she was brought back in Aug to film 2 episodes. Then she was asked back for Emily's final episode.) She even talks about how her character next season will be different as apparently
her original character was so dispensable.
For Paget, her situation was different. She knew she was definitely leaving. She knew how many episodes. Her story line had an arc and an ending. Then when ABC/CBS realized they goofed, they re-negotiated her contract yet again and said they want her to stay on for next season after all. BUT, her agent renegotiated the OUT clause, where Paget could leave if she got a pilot offer for a NEW series.
After also being so shabbily treated and told her character and talent was originally so dispensable to the studio execs, Paget took the pilot. I think she felt she closed the chapter on CM, in her life & career. Filming that new pilot was a new
beginning for her. Even though the future of the pilot was uncertain, she had moved on.
Then the ABC/CBS studio execs decide to exercise their right under the existing contract, where they can bring her back to the show if her pilot doesn't work out. According to Paget in her newest (second) Tweet, below, yesterday, they didn't even give her the courtesy of at least
pretending to ASK her to come back. Or to say, "We'd LIKE you to come back." They TOLD her she was coming back.
@pagetpaget: "Oh guys, no,I love CM. I was forced back by a contract. I ADORE the cast, crew&fans. Just hard being told what my life is. It'll be great!"
@pagetpaget: "To clarify. I HATE being shoved around. I LOVE everyone at CM + CM fans. Those are two separate things. And, yes, it IS great to be back..."
Yes, I also think, since her original contract was re-negotiated while she as also being
fired, her agent didn't have much negotiating power to ask for considerably more money like Morgan & Hotch, if they were to bring her back. (She still hadn't been realized as being that necessary. It was a back up plan on the studio execs part.) I think she is really peeved about that. So far, Morgan has the best contract. (Hopefully, J.J. got a good deal, too.
)
Here is the interview with AJ Cook (J.J.):
http://www.tvguide.com/News/Cook-Criminal-Minds-1032245.aspx