As far as I can tell, the only place the FBI puts their most wanted list up is in the Post Office. I think bank robbers are more likely to be spotted in banks, which I guess is where they hang out, so why not post their pictures there?
How did they decide to put pictures of criminals up in the Post Office anyway? Why there of all places? What are we supposed to do, write to them? Or maybe the point is that postmen see almost everyone so they should recognize the bad guys. In that case, why not just put their pictures on the postage stamps. Then the mailmen could look for them while they deliver the mail.