Integrating an LTE modem into an iPad requires a mobile data plan, and thats about it. It's not "tethered" if its built into the device.

I've always heard that patent disputes were at the root of the lack of a modem option. Apple had a prototype MacBook Pro back in the early Intel days IIRC but it was never released.

Maybe if Apple ever gets their in-house modems working, we'll see them on all of the product lines, but until then, it's a niche use case that likely isn't causing them to lose a ton of sales.