Was one time when I was in Vegas. One of the guys passing out the porn on the street was wearing BYU intercollegiate issue shorts and t-shirt. Probably an ex-player or recruit since it's not easy to get that clothing.
Other than that all fans are welcome to wear BYU gear. There have been many non-LDS players over the years whose parents are BYU fans. Drinking a beer is hardly inappropriate for them.
Oh - and those shirts and hats that hare 1/2 red and 1/2 blue are also revoltingly inappropriate