I know you guys may think this is silly but cave ecosystems are so fragile. Yes you are damaging it by just being there but they have weighed the risk vs obligation when it comes to National Park accessibility and decided that limited guided tours were an acceptable sacrifice to enrich the lives of the general public.
This is devastating and feels terrible. Don't take food inside a cave. Many caves have lockers outside. Don't eat inside the cave. Keep your bag zipped until the end of the tour and carry it in front of you NOT ON YOUR BACK.
Ugh I want to slap this person who had an entirely preventable "accident" but they probably didn't even get a ticket.