Honestly I think some people can't discern between panto where audience participation is encouraged and musicals where it isn't.
I also think that some posters have a HUGE degree of snobbery around musicals. Of course it's OK to sing along and behave badly at Hairspray, Mamma Mia, Grease because it's only a musical, not proper theatre. REAL music fans or theatre buffs are watching Shakespeare or opera. Leave the musicals for the oiks.
What also doesn't help is the show the BBC had on last weekend which I watched with my mum - something about celebrating musicals. Lots of casts performing songs from famous musicals like Hamilton or Wicked in a massive arena, and the cameras clearly showed audience members singing and dancing. You can understand someone who is not a regualr at the theatre seeing that and then booking a ticket to see a show in their local theatre and not knowing that singing along is very much not OK.