The problem isn't with that Apple calls the device or intends for it, but how the cellular carriers decide to treat the device. What does a handheld, iPhone-like device that Apple calls an iPod Touch look like to cell carriers? A revenue destroyer... unless you treat it like an iPhone and require iPhone-like plans.
Ultimately, it's the carriers that are making the decisions on what plans you can and can't get on these devices. Want proof? You actually CAN get a
data/text-only plan on an iPhone... if you manage to
convince them that the
primary user of the phone is deaf.
Even then, these plans are definitely not in the $15-20 range you're asking for though. You won't even get that in most cases for tablets, unless it's a very limited plan (prepaid 200-300MB or so, or for a limited time span like a day or a week), or as an add-on device to an existing account with a full smartphone plan.