These threads always go the same way, and it's really sad.
Of course restaurants should have the correct labelling for dishes, so people can trust the labelling and make an informed choice about what they're eating.
There is an official definition of what a vegetarian is, and eating parmesan with animal rennet in it doesn't fit within that definition. The problem with people calling themselves vegetarian, but then eating gelatine, or rennet, or fish, or even chicken (yep, there are some people out there who call themselves veggie but eat chicken!) is that that then people/restaurants think that this is what all veggies eat, when of course they don't. It's less common now, but does still happen, where a fish dish is labelled as vegetarian. Great if you're pescatarian, but not if you're veggie!
It doesn't matter whether you think vegetarians are the worse people in the world for daring to eat dairy. You're allowed to have that opinion, just as vegetarians are allowed to have their opinion that they're happy to do it, as per the actual definition of what being a vegetarian is.
No-one is saying restaurants have to cater for vegetarians and vegans, of course they don't. But most restaurants understand it's in their interest to, as it's pretty common to have a veggie/vegan/flexitarian/dairy allergy (etc etc) person in a party of people eating food, so they clearly want to cater for that in order that they don't lose any business. And that's great if they make that decision, but the people coming up with the menus/cooking the food should have enough knowledge to ensure that the things they are claiming are vegetarian (or vegan etc) are actually what they say they are.
I would never criticize anyone for what they choose to eat - if someone wants to eat meat and fish that's entirely up to them. I do find it odd when people eat meat but 'won't eat lamb because they're cute', or turn their nose up at venison because 'I can't eat Bambi', or think it's weird to eat things pigeon or rabbit etc (I don't really understand why it's fine to eat one kind of animal, but not another). However, I would never say this out loud to them, and respect everyone's individual opinion to eat whatever they like (or don't!). Unfortunately, people tend to not be so respectful back, and try to enforce their opinions on people who are veggie and vegan about how it's so terrible. I have NEVER encountered a veggie or vegan trying to enforce their views on anyone, but plenty of meat eaters who do. It's like they're personally affronted at someone not wanting to eat dead flesh, it's very bizarre.
So, eat what you like, but remember that there are definitions of vegetarian and vegan, and that it really isn't unreasonable to expect supermarkets/restaurants/catering etc to ensure that food they're offering that is labelled as such should actually be correct.