Carpenter and the ACLU say that because police failed to obtain a warrant related to that location data, it violates his Fourth Amendment right against unreasonable search and seizure. The government says it needs only to demonstrate reasonable grounds, not probable cause, to access the data.
The larger question, though, is not whether police should be required to obtain a warrant to access location data from cell phone service providers, but whether cell phone service providers should be providing this information to police without a warrant.
In other words, if cell phone service providers were taking their customers' privacy (and constitutional rights) seriously, then this Supreme Court case should actually be the government suing a cell phone service provider for not turning over customer data in a case where a warrant has not been issued.