I don't think it's to discourage people from booking. I think they want the change fee. They are probably thinking, "Hey, if we are going to give you a discount, we are at least gonna get $50 out of you for giving you a deal".
First off, the change fee isn't likely to apply to too many people. If they announced in mid-August, it would only affect people booked right at the start of October. And, if there is a mid-August announcement, it may include November/December dates, or even 2012 dates.
So collecting the $50 may certainly be part of the logic, but not the whole picture.
I'm sure Disney could give you the full curve of when most people book. But an educated guess would be that most people don't book over a year in advance, and most people don't wait till the last 45 days either.
And while the people who read this board may book, and then wait to apply a discount --- The majority of consumers aren't going to "re-book" after they have made their reservation. For a family that books 90 days out... They may never even notice a discount offered at the 45 day mark.
So Disney's logic is fairly easy to follow. They want to book as many rooms as possible at rack rate. If they offer discounts too early, then some people willing to pay full price, will be collecting the discount.
Their hope is to obtain pretty decent occupancy with no discounts, and then offer a last-minute discount to add a few more bookings. And yes, collect $50... or discourage changes, from people who were already booked.
If this is fully their logic, then it's unlikely we will see 2012 discounts for a while.
On the other hand, they may offer 2012 discounts early, just like last year ---
Following the logic of, encouraging early booking to lock in a pretty decent occupancy.... And then hold off on discounts when the majority of people tend to book. With another wave of discounts at the last minute, if necessar.