There is no restriction that i'm aware of against showing U.S. currency via any medium, be it print, tv, movies, or anything else. If i had to guess, the reason real currency isn't used most of the time is because it's much easier (not to mention cheaper) to use something else other than the genuine article. After all, props have a way of "disappearing," and that's with stuff that doesn't even have intrisic value to speak of.