[Casloy]

why can't I legally tap domestic cell phone calls them, after all, its just an extension of my ear.

Actually, you can. It's the government that can't. Back when there was only analogue cell phones a lot of people listened to conversations using commercial scanners. Now, you can't pick up phone conversations without very special equipment not commercially available.

[oceanview]

I don't believe you can legally tap cell phone calls, even as a private citizen.