I know several vegetarians who do not eat meat because they don't like the taste/effects of it on their body. I don't go around interrogating vegans as to why they are vegan, I merely note this for times when I may host them and work out what they can drink/eat and what I can realistically provide with my energy/capacity. Respectful honest conversations about that are what grownups DO! Just like I would if someone ate Kosher or had intolerances/allergies or even strong food dislikes.
No one has any right to put something in someone's food which they have said they do not, or cannot eat. If someone requests something which seems to contain the unwanted item the individual or catering establishment should say "Err, this contains X" but if the person chooses to go ahead with informed consent then that's their choice. It's highly unethical for anyone to sneak unwanted substances into food as some kind of revenge for perceived over-fussiness - for catering establishments it's also likely to be highly unlawful.
And while I sympathise about "fussy" food people who contradict themselves, it's not a justification. I have a friend who told me there were various things they could not eat. Significant effort was put into providing suitable alternatives so they were not left out of treat foods. On the day they asked to eat the regular version - I told them this was full of X and Y which they said they could not eat and that we'd got a special version - they wanted to eat the regular version anyway, so I let them. The next day they got ill and complained that I must have contaminated something else, so I pointedly reminded them about eating the thing I had told them was full of "unwanted" foods so they had to take responsibility. That person was being annoying, they haven't done it again since as I think they did realise they were being an arse - boundary enforced. The majority of my friends who have "fussy eating issues" of any kind are not like this, they're usually appreciative of effort and are grownups about it...
In the case covered recently It was later clear that while the initial understanding had been it was a vegan group, it turned out only some were vegan, others were vegetarian which is why a mozarella pizza was ordered by someone. To have put probably-not meat but probably-dairy into the food of other people in that group (who were probably vegans) as revenge is appallingly horrible, spiteful and dangerous. One of those vegans could have been lactose or dairy protein intolerant and in not wishing to eat meat described themselves as or even been vegan by preference. They may have had other reasons for not wanting to eat dairy which should have been respected no matter how weird. They shouldn't have been given contaminated food because someone else in their party or their party were perceived to have pissed off the chef.
I don't condone death threats made against the chef cos no one deserves that no matter how spiteful and nasty they are. However if she's spiked food this time, there's a high chance she's done that before to someone else. She doesn't deserve to work in the catering industry if she has so little clue about ethics, safety and the law that she lets some "challenging" customers annoy her so badly she contaminates their food. I wouldn't go to an eating establishment which was known for not respecting people's food choices - EVEN the really fucking annoying inconsistent ones... That's just basic catering 101.
And yes, I'll totally agree some vegans are fucking annoying, sanctimonious, inconsistent and full of bullshit woo! <--- unPC opinion. (But I'll still not contaminate them! Just bitch about them online!)