The Rams simply made a prudent move. He was going to be a $5 mil cost even if they outright cut him.
By restructuring, the Rams made it a no-lose proposition for themselves because they don't have to pay him the additional $3 mil, bringing his cap cost down to the $5mil and being able to keep him anyway.
Austin is certainly worth a roster spot. He is better than most any 53rd player on the roster. So since he was a sunk cost anyway, what's wrong with trying to get something out of him? Lord knows he's been a good trooper. No trouble to the team. Not screaming to the press about not being used enough.
And there's been a lot of rumblings that he had a variety of injuries last season that hampered him all year. I don't know the truth of it, but that smoke just won't go away.
He doesn't have to go off too many times to change a game. He's one of those kinds of guys with his playmaking ability.