I think because you have tethering, that is why the tethering option on your iPhone is enabled. When you don't pay for tethering, or get it for free, that optio

I think because you have tethering, that is why the tethering option on your iPhone is enabled. When you don't pay for tethering, or get it for free, that option is disabled. On the Nexus 4 and 5, the option is enabled regardless if you paid for tethering or not. This is because the phone comes directly from Google, so t-mobile did not get to install bloatware (custom OS) that disables the feature. I call it "native" because it is built into the OS, versus installing third party apps that mimic the feature. Usually these apps require rooting your phone.