The 4th Amendment is at issue here. This is just a neat end-run around the 4th Amendment; it doesn't apply if you can put enough soft pressure on businesses to have them report on their own users.
You're right that it's legal under the current interpretation of the laws. It just means the 4th Amendment isn't worth the paper it was printed on, and it'll get relegated to the "stuff people used to care about category" like the 3rd.