I like the rule personally, you can't just grab someone from behind, and throw your body into the back of their knees that's just dirty, and personally I've played football and have seen millions of games, I've seen very few tackle this way.
I've never had a guy grab me from behind, and swing his entire body into the back of my legs or he'd be in a wheel chair today.
All Roy had to do was grab the guys legs and wrap them up, he was in position to do that, but chose to reach for the back of his collar. The legs were right there for him.