..if your device only supports cellular networks, it's probably a phone.
I can't think of too many internet-type devices without either bluetooth or wifi.
You missed the point. In my home, I have one landline and four handsets. I only pay for one landline, and I do not pay a per-handset fee.
I have one broadband connection and too many Ethernet and Wi-Fi devices to list. I only pay for one broadband connection, and I do not pay a per-connection fee.
I have two mobile plans and two mobile phones. I pay for each plan and phone individually. I should only have to pay for one plan. I'll concede an additional charge for additional phone numbers.
I think the same reasoning should apply to cable and satellite television providers as well, but consumers have blindly accepted that per-device fees are OK.
Ironically, the per-device fee is one of the many things that landed AT&T in front of the Department of Justice on antitrust charges in the 70's. "Adequate" competition exists in the mobile space, however, so it's unlikely to change--unless someone puts together conspiracy charges re: handsets, early termination fees, etc., since all providers do pretty much the same thing.
EDIT: And "it's probably a phone" is a bad assumption. There are many 3G data devices on the market that do not function as telephones.
