Well, yeah, but the parent post talks about the police calling the number on the sign and using that to get a conviction, so I assumed that some sort of other crime was being committed, like maybe they steal the caller's bank info or sign them up for some scam service.

The reason the police don't do what the parent says is not because of difficulties with attribution of the act, it's because they care so little about the offense that they're not going to expend even the slightest effort to prosecute it. If they saw someone putting out signs, they might tell them to stop. Maybe.