Oh, that would SO infuriate me. We plan our day around when we can check in...book lunch somewhere (when staying at BWV, we always eat in Epcot, then if our room isn't ready before 4, we just head to BWV for 4 o'clock). The only issue of "by rights" is that I have a right to get into my room at 4 PM.
Does your DD know that checkout times exist? And if so, why didn't she know WHAT it was? My DD is in college and taking trips on her own now with friends. We haven't let her use our points (we need them

), but just by traveling with us over the years, she knows that there
IS a checkout time.

She went to WDW in January and stayed at one of the All Stars, and when she was showing me all her plans, I commented "Ugh...you have to check out really early" when I saw that she had early plans for a park on the last day, and she certainly realized that. If a college student is old enough to be taking the trips, they're old enough to know the rules.
I hardly think of being charged an extra night as giving anyone the right to the room; it's a penalty. They need to enforce some "incentive" to follow the rules. The rules are there for a reason...they need to turn around all thse rooms in a short period of time. If two people did that...checked out late...and one room had another party moving in, and another didn't, the one that had another party moving in should have to pay, but not the other??
Altho I may try to pay cash rather than use points, whatever the decision...I'd chalk it up to experience.
Reminds me of a recent experience for a friend of mine. She rented a car through priceline for her next trip; cruising first, then a few days at Disney. Then she realized she had her son's college graduation date wrong...and they wouldn't be returning home until two days after. So, she changed her trip to doing WDW before the cruise, and called Priceline to see if she could change the dates. She can't. And altho it's kind of aggravating for her to know that it's not like she's backing out of renting through priceline...they'd still be getting her money, but just for a different date...she's out of luck. They won't do it, and she can't argue it...the rules are the rules, and she knew that going into it. (And even if she didn't, "ignorance of the law...") It is what it is.