Because he sucks.
Seriously, that’s the reason. Williams has enough time that he doesn’t go to waivers, he’s just a FA. And since he sucks, no one is going to offer him a big contract. So he’s completely safe to release.
After the waiting period they will bring him back on the PS, then over the next three weeks they will play the call him up/send him down game. By the time they have to open a roster spot for him in week four someone will likely have been hurt and gone to IR, so they’ll be a spot. (And in the happy event that doesn’t happen they will have a better idea of who they can live without.)