They don't block it, they charge for it. The devices are capable of being a mini wireless router, providing Wi-Fi access to other devices. However, since the "service" you pay for is for your phone device only, you must pay a premium plus any activation charge for other devices (your laptop) to use the mobile Wi-Fi hotspot feature.
Yes, you may tether the device, but even then you need to be aware that you may be breaking your carrier's terms of service. Disputes abound on the web regarding this topic and whether or not carriers can tell if you have "rooted" (or "jailbreak") your phone and/or running a tethering app.
It would seem as though carriers are concerned about their bandwidth consumption where as it probably comes down to the almighty dollar.