It isn't. If the police are investigating John Smith and they get a warrant for the files of John Smith then they don't also get the files of anybody else along with them.

And importantly, there has to be sufficient reason for investigating John Smith. It can't be arbitrary (he looks funny, has a limp, is black, is gay, plays Doom, is a Muslim, etc). Rights can be infringed, but they need reason. And they need good reason.

How do police find out that John Smith is the person whose files they want to get a warrant for?

Maybe because John Smith was one of only eleven people who signed in to a building on the day a crime took place, and he signed out right after the crime happened.

But should the police not look at the sign-in sheet at the building because that will infringe the privacy of ten innocent people?

> How do police find out that John Smith is the person whose files they want to get a warrant for?

The victims go to the police and tell them that John Smith stole from them, so the police go and seize his files to confirm that the victims are telling the truth.

> But should the police not look at the sign-in sheet at the building because that will infringe the privacy of ten innocent people?

Asking for the sign-in sheet and seizing the sign-in sheet by force against the wishes of its owner are two different things.