Just a motivation ploy. However its not like the defense at this point deserves to wear them. AT ALL. Lets not forget what it means to wear those shirts. If they want them back, then earn it.
Actually, historically, pre-Pelini, pre-Callahan, what it meant to wear a blackshirt was that the wearer was first string defense. No more, no less.
Granted, in that time frame, we had many great defenses that played with a nasty disposition. I miss that as much as anyone but, let's not misrepresent what and how they should be awarded and treated. This is exactly what I don't like about the way Bo has chosen to deal with the blackshirts. It's just a punishment because they aren't good enough. It totally ignores the more significant aspect of motivation and working your a$$ off to earn one.
I made a big pitch prior to this season to get back to doing it the old way. Problem is, even though he handed them out before the season, the whole tradition of it is treated differently by Bo. I'm now of the opinion that, as long as Bo is the boss, they should box them up and forget about them. He's doing more harm than good with them. He needs to first worry about teaching these kids fundementals and get them in the frame of mind to run through walls for him.