They can put default APN configurations in the SIM card that (by default) would have different fares for mobile usage and tethered/hotspot usage. They cannot block you from changing the configuration, but I think in theory they could terminate the contract if you did so. In practice 99% people got used to buying both a "computer SIM" (which is typically sold with a cheap Huawei USB modem) and a "phone SIM", so the providers that do differentiate that way don't bother going after violations.