The teams cut guys during contracts all the time, so for the same reason a great player like Donald who played out four years of a six-year deal and wants a new contract I see no problem with it.
This is all bluster anyway...negotiating 101, use whatever leverage you have to get what you think you deserve or are worth.
If Donald retires now the Rams go from a SuperBowl favorite to a playoff team who maybe gets to the divisional round. He is using the leverage he has to get what he is worth.
I have no issue with it. If you think for one minute the Rams wouldn't cut ties with him if he was too expensive and not that good with years left on his deal...
Look no further than Gurley. Its a two way street and Donald has every right to do what he is doing...More power to one of the best Rams to ever put on a helmet.