Tricky one that, some bloke!
Can certainly recognise the scenario, that some people do come to festivals for just for the sessions, but on the other hand, the pubs where these sessions occur do well out of it, in terms of pints bought and meals ordered, and some of them, give them their due, do become festival sponsors.
At our local festival we do try to produce a list of pubs where sessions might be welcome and at what times, as well as putting this info on our website, and signs for the publicans to put in their windows - "Musicians and Singers Welcome". There is, in any case, an overlap between those who do sessions only and those who come to some or all of the main events.