I think that in order to get Obo on the 3 week IR, the Rams decided to carry him on the roster.

The LA Rams can re-sign Justin Lawler after a roster spot opens up if they wish to do so. Or add Lawler to the practice squad, where he can be elevated twice during the season without penalty.

I believe the NFL rules had a part to who the Rams went with at outside linebacker for the initial roster