When you say something like this
It makes people think that you are tethering your computer use through your phone, thereby raising your internet use and causing even more strain on the network making everyone else around you slow down. If you are using just your cell phone to internet browse, e-mail, stream, etc, that is normal usage and yes, you are correct, you shouldn't need an ISP. Also, if you are paying for the capability to tether your computer to your phone and use internet connectivity and fall within all the guidelines imposed with the tethering agreement, again you are correct, you don't need an ISP.
Many people, including myself, have wifi at home and use that to connect the smartphone when we are at home. I get 30mbps from my home internet, so my smartphone flies through any task while I am connected to wifi.
There is no way I could cut my internet service, as I am dependant on it, not for cell phone use, but for all the other gadgets I have around the house that use internet. Connecting my cell phone to it is just a plus for me, and it helps out those around me because my usage is no longer on the network.