i have been denied access to restraunts before for wearing hats.
as long as no-one can wear them, then no prob. ever see the signs that say we reserve the right to refuse service to anyone. and in one case that this happened to me it was at h.d.hotspurs, ironicly my group of 14 were meeting there before going to the rodeo. all of us being cowboys and riding in the rodeo, we all had our gear all on.
now if a cowboy has to remove his stetson at a western bbq joint, then i see no reason a sikh doesn't have to take off his freaking rag. religion or not, if they have it posted that hats are not allowed, or for that matter enforce the rule to everyone, a hat is a hat, rag, yamika, stocking cap, or stetson, take it off or get the hell out of dodge.