Because it's actually your telco's phone. They're the one that has the license to run the baseband computer and RF transceiver. The 'pad' computer device is sort of yours. But there's no legal way to have ownership of a cell phone unless you yourself bid for and get the RF spectrum and set up your network in a way that accomplishes the FCC coverage and timing requirements. Then run your own telco for your phone. Basically, impossible.

Smart phones try to limit and firewall the interface between the two but tight integration is required for energy efficiency. So a smart phone, or a cell phone, can never be yours. They aren't good choices for doing computing and this legal reality is becoming more and more obvious with time.