http://www.amerilawyer.com/trademark/tm_tradedress.htm
Trade dress and trade marks are not the same things as copyrights.
Another clothing store is seeking to capitalize on the 'value' of the number 22, as it is associated with AF. The same way McDonalds 'owns' the colors red and yellow and the arch, AF contends it 'owns' the number 22.
Now, the fact that it's a number, they never registered it and that AF has sued people in the past (unsucesfully) for producing 'knock offs' doesn't make a great case for them, but it's not some company going sue-crazy.
These laws actually protect both the companies in letting them build value into the brand, and the consumers, so companies can't attempt to dupe people into purchasing their product or service.
Also, if a company fails to protect their trademarks (or perceived trademarks) then they become invalid. Asprin, cellophane, and escalator are all brand names. Or, at least, they used to be. Ever own anything made out of lycra? That seems to have become generic too; it's a brand-name for spandex.
(IANAL

)