It's more straightforward than that.
I have a constitutional protection against the government searching my stuff without a warrant. However, the constitution only restricts governmental actions, not the actions of citizens or other private entities.
So, the government can't search my records without a warrant -- but if a private entity has any data on me whatsoever, there's no prohibition on that entity providing it, and law enforcement can ask (as opposed to demand) anything they want, just like any of us can. All they do is ask that party for the data and if that party wants to, they can provide it.
You don't have to agree to this in any contract, T&C, or anything of the like. Private entities have the right to do anything they like with data (or property) they legally control by default.
You'd have to have a contract with them that explicitly prohibits them from sharing. In other words, it's opt-out, not opt-in.
Legally, it's no different than cops knocking on your neighbors doors and asking them if they've seen you doing sketchy things. The neighbors don't have to comply with the request, but most will.