I think people in the UK put up with crap sound in venues because they don't often experience better.

Most bands would kill to play in venues like Musikzentrum in Hannover, yet the best I've seen in a while in the UK is XOYO near Old Street.

I like to think that if enough punters boycotted bands playing in crap sounding venues the promoters would insist they upgraded their sound systems, but most likely promoters and venues would go out of business.