I've never heard of that, but I'm in the US with Verizon and fully trust they'll either want $10/mo (because it's a device) or $20/mo (if it can make phone calls).
Well Verizon is a special kind of "evil" from what i can tell.
In most places that's not as balkanized as the US carrier market, you basically get multiple SIMs for your plan and that's it. Nobody gets to question what kind of device you put those SIMs into.
From what I've read I've always got the impression that cell networks are one of the things the US is furthest behind in. Even when we are more or less technologically up-to-date, the customer experience part is still terrible.
In Poland you can get a "sub" simcard that will use your cell/data plan from the main one, for just 10 pln/month (sometime ago it was 5 pln/month probably some of the carriers can have it even cheaper) ~ 2.8 $/month.
It is used mostly for tablets here(main simcard is in the phone) or a mifi for car.
See I'd happily pay three dollars a month to get my iPad on the network. That seems like a reasonable fee.
I don't expect to use my iPad (or anything else) away from Wi-Fi all that much, so that may be part of the issue. If I thought I'd use it more $10 may seem more reasonable. Like if I traveled a lot.
In most places that's not as balkanized as the US carrier market, you basically get multiple SIMs for your plan and that's it. Nobody gets to question what kind of device you put those SIMs into.