Interesting question.
Is it illegal in the United States to stand on a hilltop and use a telephoto lens to photograph people who go topless in their own backyards?
In France, this is a criminal offense, "atteint a la vie privee" (an "attack on private life"), but in France, individual privacy is a legal right.
But is it illegal in the US, in California specifically, as well?
Apparently. After all, this law firm has filed a suit. Assuming that the suit isn't completely frivolous, there appears to be a right to not be photographed on private property.
Yeah, there are library shelfloads of cases and law dealing with this.