Chop blocks below the knee are absolutely not illegal. Crackback blocks are almost never called, but most times they're called in context of where the ball is snapped than in the open field. Chops between the waste and knee (but where the knee starts is much discussion, most refs call it even slightly above the knee) are 100% legal, and its not only within the tackles.
Bear rolls, leg whips, and chop blocks below the knee are always illegal, and that's what he did. He hit low, and he rolled, but he looked more lazy than he did malicious to me.
When I played, which wasn't that long ago (relatively speaking...relative to moses), it was aim for the hips, with the understanding that you're gonna hit slightly lower, but almost never illegal.
However the most I watch the play the dirtier it seems, as it looks like Farve intitiates the roll long before he even makes contact. It does look pretty awful actually, it looks like he never really meant it to be an honest hit, as in he never wanted to risk his shoulders, so he starts a roll way earlier.
I swear I definitly remember playing football for like 10 or 11 years, I got t-shirts and letterman jackets and hitch in my walk (from an illegal chop block, or so I tell people) and everything.