
I don't know, OP, but I have hired a pi to keep an eye on this thread in case any answers turn up.
(Edited 7 minutes later.)

I think it's because "stalkers" cross a line that a PI knows he can't legally cross. For example there is nothing wrong with "following" a person and "frequenting" their known hang-outs and "watching them" in public, but PIs don't normally, i wouldn't think, need to "contact them" or "root through their garbage" or, you know, "go in their house."
But yeah, you can photograph people in public and do all the PI type stuff as long as you don't, you know, "stalk" them.
(Edited 48 seconds later.)
@302,377 (squeegee !firstkPE1Q)
PIs will go through your garbage. Anyone can. Once you throw something away, you relinquish possession. That's why cops don't need a warrant. Never throw anything away that can implicate you in anything. Burn it.
@previous (F)
This is true. Since I just learned about this in my Civil Rights & Liberties class I will be a know-it-all and inform you that the Supreme Court decided that in
California v. Greenwood.