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.
Sprint segments their users as non-smartphone vs smartphone (+$10/mo)
Other carriers segment their users as non-tethering vs tethering (+$50/mo)
Non-smartphone $15 Smartphone $25 Smartphone + tethering $45