Those that are OVO state that on the flyers anyway? if it don't say OVO can one assume that boot's carvers etc are played? On that basis, people pays their money and take their choice.
Maybe there should be a OVO logo that people slap on their flyers, a bit like Abta, Corgi and the like?
At the end of the day, it's a (supposedly) free country and if I wanted to start up a night, in direct competition to someone 2 miles up the road and was just going to play cd's, then no one can stop me...... not that I would want to do such a thing, but it would be my freedom of the market to do so.