Maybe they are (based on this and some other reports) finally closing the FP+ from 4-7 loophole for those being allowed in early to beat the crowds all trying to enter the party at 7 p.m. WDW lets people in early to help them out and wait less time to enter, maybe they think getting fastpasses during a time that isn't even included in the party ticket is going too far. (I know the system lets it happen, but cars would let you go 125 mph, doesn't mean you should drive that fast.) I know I am to leave by 7 p.m. and I'll make sure i'm not there a minute past on a party night if I don't have a ticket. I won't even get in a ride line, eat or shop that would cause me to in the park past 7. However, I'll be going to Guest Relations when I arrive to ask why it's okay to limit the number of fastpasses available to me as a day guest from 4 p.m to 7 p.m by letting early party entrants have access.
I do feel for those that have their existing plans impacted however. There are also reports of ADR's being cancelled for day guests who tried to dine too close to 7. The timing and exchange between the day and the party needs to be ironed out, but it also needs to be clearly communicated. And it shouldn't matter what the fastpass was for. Just cancelling it for one attraction and substituting a "lesser" attraction doesn't make sense to me. Either its ok or it isn't.