I mean... yes, that's how police surveillance works? People don't want to do illegal things when they know the police is watching, so sometimes the police has to spy on people before they can prove they did something illegal. The person being spied on can't present their side of the story, because if you tell them they're being investigated, they'll just lay low.

So yeah, there's always the possibility that the cops spy on someone innocent or try to dig up dirt on a journalist or something, and that's why warrants exist. If you don't think a judge's oversight is enough for the police to intrude on someone's privacy, then you're basically saying that the police should only ever have access to OSINT sources and nothing more.