This is a bizzare situation for sure, but all indications point towards the Lions not wanting him on the team, in which case it makes sense that they don't want him to get hurt and then be stuck with an injury.
Its all conjecture at this point, but IMO either Mayhew screwed up the contract which makes it difficult for us to dump him at this point, or Quinn wanted to keep Tulloch, but at a reduced cost, which Tulloch and his Agent promptly rejected, with the assumption that the Lions would cut him in order to avoid paying the $500k roster bonus, and then they he would hit F/A. However, Quinn said "Nah this isnt how I role" paid him the $500k, and locked him out of F/A. Its basically a lose lose situation at this point, the Lions had to pay $500k for a player they dont want on the roster, and Tulloch is now basically going to be playing for the vet min. this season.
This is just my theory - which is probably wrong, but its all I got....I doubt we will ever really find out the truth.