The ACAB tattoo is associated with certain right wing and criminal groups. And whilst the individual may not be offended by it, it makes it no less insulting to people from cultures where this is a factor. It would be similar to having HH or 88 tattoos. I very much doubt that such things would be allowed, so I am disappointed that this is.
At least in my area, it's used in the opposite context you're mentioning, specifically by certain left-wing subcultures, where it serves as an acronym for
All Cops Are Bastards. With a few exceptions, these individuals aren't criminals at all; they're mostly just punk and revolutionary youth who, at most, spray walls or abandoned trains with this, among other slogans.
Unless this acronym holds a far more dangerous meaning elsewhere that I'm unaware of, and
if the model actually intended it in that specific context, I don't see any reason to make a fuss over it.