The University website does give a reason for not allowing abercrombie & fitch clothing
"Abercrombie & Fitch and its subsidiary Hollister have shown an unusual degree of antagonism to the name of Christ and an unusual display of wickedness in their promotions. In protest, we will not allow articles displaying their logos to be worn, carried, or displayed (even if covered or masked in some way)." http://www.bju.edu/prospective/expect/dress.html
While there are other companies who have similar adverstising A&F seems to have had the most press about it from parents, etc which is probably why Bob Jones U has named them specifically.
As Christian, rasied attending a conservative Baptist church, I that I feel I have never blindly followed rules, etc such as some may see the students at Bob Jones doing. Sure as child I did what my parents told me to as far as what I wore, going to church but as I got older I looked for a way that these Bible-based ideas applied to my current life and chose my beliefs and live style from there. I would venture a guess that the students attending Bob Jones have done that same. Their choices may be different from ours in many ways but is that really a reason to call them out?