Actually, I didn't take your post as such, but that you for mentioning it.
From what I've read, the computer system will not allow an agent to shorten without rebooking. Perhaps they turned the feature "on".
When you were there they probably were talking about it regarding stay controls, but I suspect it wasn't that detrimental to Disney. I did it all the time - booked when the discounts were announced and then subtracted if I had to. Actually, more often than not I had to add since I wasn't able to get the full length I wanted on the first try.
FP+ has definitely magnified the problem. Everyone wants an advantage to get the popular rides, and well, with only 3 bookable in advance per day, frankly, I don't blame them. That is my biggest gripe with the system. That, and having to decided where I'm going to be in advance of getting there.
I actually didn't run into the problem - my flights changed and I extended
I rarely subtracted.
This policy was very poorly implemented and done so retroactively. That is patently unfair and supervisors have to become involved to override the system. It would have been far easier to cancel all the fastpasses of those shortening within 60 days. That would have stopped it dead in it's tracks.