I wouldn't mind, and agree with you, HOWEVER...
When you auto-picked your team before the changes, the game EXPECTED a full back in these positions, it would select a 70 DR or DL over a 90 DC every time.
So people like myself went out and bought full backs. I have (only including 88+'s) 2 DC's, 2 DR's and 2 DL's in my squad. Now I am desperately short of central defenders, and have to rotate in a 70 to give my guys a breather. Had this change been implemented slowly, then great, but I wasn't aware it was coming in until the day it happened.
There are still some issues of the game showing a player in a 'green' position, but getting the error of them not being in an optimum positions. Such as formations with 3 AMC's, where the tactics screen (wrongly?) shows wingers in the wide positions as green.