Interesting. It's not exactly trespass in the conventional, common law sense. There was a case a while ago where an estranged ex-husband projected some less than savoury messages about his ex's various infidelities on the building she worked at (and later apparently her home). He was arrested for trespass but the case was thrown out. I can't remember how the case was ultimately disposed of.
There are more and more cases like this of using these projections as a form of protest, so ultimately it will be interesting to see how law enforcement and the judicial system sort this one out. Public nuisance laws maybe? It must violate local ordinances or codes in many cases, because I know that for a business to do their own projections onto their own buildings they typically need to get a license to do so. It's also possible that the equipment itself might block a public sidewalk, etc, enabling the police to intervene.