It's because apparently the word 'Nike' sounds like the word in Arabic for sex, which is something like 'neek'.
Supposedly, ISIS gave out leaflets letting people know not to buy Nike products. If this rule is broken, the perpetrator can expected to be imprisoned or whipped.
This is not the first clothing item that ISIS has banned. They've also banned jeans for giving an 'attractive' and 'sexual' look.
The other reason Nike might be banned is because it is named after the Greek goddess of victory, which is certainly against ISIS standards.
That being said, it's probably safe to assume that Nike isn't too upset over this, as the company probably doesn't want to be associated with such a group.