Earlier quoted context omitted.
Yes you are. Social organizers, advertisers, trend setters, ect frequently get free product, kickbacks or some money for their 'work'. That's the work you have done. It's not your intent to do work and it's probably not the pub owners desire to pay for meetup organizers, but regardless of your and the pub owner's intent, work has happened.
Is it not possible that the situation is a win-win for all parties and that all parties feel like they are well-compensated in terms of the value they've received, and that therefore no money actually needs to exchange hands? You may as well ask why people volunteer their time at all.
It's fair that both parties feel like they're benefiting, so no direct transaction needs to occur.
Comparing this to volunteering generally isn't the same. I volunteer with local registered charities because I believe in their mission, and the volunteer work I do is directly impacting and improving the lives of the people I'm working with.
Which may be the crux of the GP's question. Why volunteer in the community in a way that results in a for-profit organisation monetising you for their gain, instead of finding a non-profit in your community to be volunteering your time towards? The non-profit / charity in your community's goals are more likely to be aligned with your own, than the for-profit social network.