It's all about marketing and aiming for your target audience I suppose. If they're gonna call themselves a blues and rock n roll bar, guess what kind of music they'd want to have playing on stage?
Why, blues and rock n roll of course. Well, subject to interpretation and definition.
At the end of the day, their obligation is to their own fiscal bottomline, not to the guitar players. If they feel that they can get enough butts in the chairs drinking beer even with the genre restrictions, that's up to them. 8)