"The standard argument in favor of unfettered government purchases of private data is that such data is commercially available, and so anyone should be able to purchase it, including government officers."

Agreed.

"... while government officials can generally purchase items available to the public without constitutional restriction, sensitive private data about cell phone users isn’t actually available to the public."

What? I may be misunderstanding, but this looks like a deliberate attempt to conflate two concerns: publicly available data for sale and sensitive private data. I don't see any argument here against the government buying publicly available data.