Same trouble here. We called today (60 mark) for MK Plaza at 5:30 on 9/13. Quote from the DU agent: Latest they have is 4:30.
I think the OP is correct. With the economy and WDW cutbacks, it makes sense. Open the restaurant, fill the early seatings. Then only when those get to a certain % full, open up more tables later in the evening. That way if there is low demand they can close the restaurant early and save $$. The other approach (like in the past, get anytime you want from the beginning) may lead to a fairly empty restaurant all evening, which is bad business. You lose money having to stay open, keep food hot, and keep waiters in the place who aren't working.
I think it's terrible, and most likely they will be open much closer to closing time (I mean, MK on a Sat night, when it's the first Spectro in over 2 weeks, come on, MK is guaranteed to be crowded), but the best we can do is change the PS as our arrival gets closer.
Just my $0.02 . . . I hope I'm wrong. I'd certainly like some confirmation from someone in the know.