I still don't see the problem here. The only things that aren't subpoena-able are things covered by doctor-patient confidentiality or attorney-client privilege

I still don't see the problem here. The only things that aren't subpoena-able are things covered by doctor-patient confidentiality or attorney-client privilege. My phone calls and credit cards and bank accounts are similarly subpoena-able. The privacy issue isn't that this stuff is made available to investigators through due process after they already have reasonable suspicion, it's when investigators just go on a fishing expedition through it.