What you are looking at is his ability to fight off a block and cause a RB to slow down and change his path. I know that the negative folks see that as not doing his job. However. in the 3-4 the Job of the DL is to do exactly that setting up the LB to make the play.
IF a D Tackle manages to make a tackle for loss that is a bonus. It is not however the expectation. realize they are managing 2 gaps. vs 1 in a 4-3. No one passes it up but if you know anything about the 3-4 the DL are meant to be disrupters not play makers. LB are the play makers.Can't imagine any defense designed to pass up on a tackle for a loss.