NFL probably wouldn't of fined Matthews if the QB didn't get hurt. Those hit happen all the time and the NFL normally doesn't do a damn thing... unless someone is injured or if the a team or media are bringing it to the meida attention.
I believe that is the way some teach how to tackle. That's one of the ways to bring the most force into your tackles, and NFL normally doesn't seem to fine guys for that type of tackle unless it gets extra attention for some reason.
And just because the NFL does something, that doesn't mean anything. I still remember the NFL STUPIDLY apologizing to the Vikings for ref getting the WR Harvin's catch wrong (When CLEARLY the ref was right on that one)... as the ball CLEARLY hit the ground and the ball didn't come off the ground like Harvin's went down with the ball, so CLEARLY the ball had moved while on the ground.
Making that STUPID move even stupider, was that Quarless earned a touchdown by catching a ball outside the endzone.... and the NFL never said a damn thing about the refs getting that play wrong. But the Vikings owner was livid about the Harvin play... so the NFL played kiss ass and through the ref under the bus, when the ref was right on that call... (which is sad, because they easy could of pointed out the ref was right on that play and wrong on the Quarless play, but that would take extra work of doing your job correctly)
In other words, the NFL isn't doing what's correct, the NFL is doing what it thinks it has to to keep the NFL owners happy... that's it. So QB got injured... of course you got to look at it and try to fine the player for the hit to try to keep the owners happy.