Cause that's the only way that the bandwidth of "a system originally designed to transmit voice" is going to limit the data rate.

The reality is far more interesting: 3G, LTE, and 4G are integrated data streams, up to 1 gigabit/second for 4G (I'm still scratching my head as to why that doesn't violate the laws of physics, but it's an international standard, and that trumps physics). Voice is just one kind of data they carry.

The fact that they're the same service for voice and data is why I view paying for a separate voice plan as fraudulent: You pay $50 to use 1% of the available bandwidth in a voice stream, then another $100 for a data plan to use the other 99%. It's not that the data plan is a great deal, it means that you're getting ripped off by the voice plan for paying $50 to transmit so few bits.