Mostly poor manners / not cleaning up after themselves. A few years back there was a family from the UK visting New Zealand that was so poorly behaved that their exploits were being tracked around the country in the news and they were eventually deported [1]. This article also cites a woman from France defecating in the street when her camper broke down. And examples of foreign tourists not respecting local traffic laws.
Not saying that this is representative of all van-lifers, but when you are traveling around in a van with no plumbing or trash disposal services, it does require a bit more responsibility on the part of the travelers to follow the rules and clean up after themselves vs. staying at a hotel and using public transportation. And not everyone is up to the task it seems.
Why would they pay hotel or property taxes when they do not reside there, and have none of the rights of residents (e.g. school system access)?
Also, isn't blaming "an increasing number of people living in vans" a perfect example of shooting the messenger and ignoring the message?