That depends on your definition of "can't." Technology-wise, yes, if the phone supports it. Terms of service-wise, no, but that's true of the smartphone plans also, they've just never enforced it before.
Right, that's how I'm pointing out that Sprint's approach is different. They just charge all smartphone users an extra $10/month and don't care whether you tether or not. Andriod phones come with the mobile hotspot app preinstalled.
Sprint segments their users as non-smartphone vs smartphone (+$10/mo)
Other carriers segment their users as non-tethering vs tethering (+$50/mo)
Sprint segments their users as non-smartphone vs smartphone (+$10/mo)
Other carriers segment their users as non-tethering vs tethering (+$50/mo)