I have been to MANY gatherings where it was stated up front that all food was to be vegetarian. The KlezNorth weekend klezmer event does it because they can expect a fair number of the attendees to be Jewish of various degrees of observant-ness, as well as some actual vegetarians, so it's a simple compromise. (Doesn't help me a lot as my religion involves continual sacrifices to a cardiac stent and a wheat allergy). You would find the same at events where you might expect a number of Muslims, Buddhists, Hindus or Rastas.

This is not some new fad, it's been routine at least since the 1970s. Most people have had most of a lifetime to get used to the idea.