I've got a Nexus 4 running the same version of Android as you, using T-Mobile's $30/month plan (unlimited data, 100 minutes/month talk) and my laptop can connect to my phone via WiFi easily, but my plan won't let the laptop connect to the Internet--that's another $15/month.
I bought the device without going into the details of all the terms & conditions
Perhaps you should do that.
T-Mobile is just doing dumb user-agent sniffing at their proxies. Install a user-agent switcher extension/plugin in your browser and identify yourself as a mobile browser and the "please pay for hotspot" message goes away. They weren't disabling anything on your phone.
This isn't related to OP's issue. The Nexus 4 has a button to enable wifi hotspot, his Nexus 7 doesn't. That's an Android thing, not a carrier thing. There are hundreds of forum posts about it -- Google did this -- Google shipped a tablet with Android's wifi tethering feature disabled.
I've read it all over the web that Nexus 7, by rooting it, can support tethering. How can that be a 3G plan related feature? I've read that Nexus 4 has wifi hotspot settings enabled but Nexus 7 does not.
Once trust is established, people tend to have faith in a company. I doubt many people read Gmail's terms of service before using it.