I think the issue, really, is a philosophical disagreement. I think that if the police are to be able to investigate crime, they need to have some invasive powers. But they also shouldn't be trusted to use them without controls. The subpoena (and warrant) process is the control we have in the US. I don't think the existence of this ability is, itself, a serious violation of rights.

Other people may legitimately disagree.

There is also a serious discussion that could be had about whether the controls are sufficient, but that's a procedural issue rather than a philosophical one.

Not if it ends there and cops make the right decisions. However it's a legitimate concern that an overly broad police investigation, even after a real crime happens, might lead bad/lazy/dishonest cops to wrongfully acuse someone unrelated.