Because it looks like they are smoking in non smoking areas. That's it. I can't comprehend why that is so hard for some to understand. People are offended by others smoking in non smoking areas. Deny it all you want, but it looks like smoking. There are busy body smoking police in the US who will cause a scene if they think someone is smoking in non smoking areas.
They are bothered by smoking in non-smoking areas because ... smoking.
Smoking produces smoke, which smells, can trigger asthma and allergy attacks, and people don't want their kids around it.
This doesn't apply to vapor. You can tell the different between smoking and vaping from 10 feet away. If busybody smoking police want to cause a scene over what is not smoking then they are stupid and will embarrass themselves. Not my problem. You are free to refrain. There are many places where I don't vape out of common courtesy, and in truth vaping is banned everywhere smoking is, where I live. That doesn't make it right and I may or may not sometimes frequently always vape in places where the ban is stupid, like outdoors at the park.
However when I visit my sister out of state where there aren't all these prohibitions I don't feel obligated to do my "not smoking" in places where only smoking is allowed. I do my "not smoking" in places where "not smoking" is ok and it isn't likely to bother anyone.