Quite simply, that's just how your taxes are assessed. I also have per line and per account charges. Moreover, Taxes can be imposed at the federal, state, local, special district, and other levels. T-Mobile will never give you a straight answer about tax assessments (believe me, I’ve tried). If you really want to know exactly how all the taxes are being assessed, you will have to do a fair amount of your own research.
Taxes for wireless are pretty weird. In most places, your account itself is taxed, and then each individual line on the account is also taxed.
I can't really tell you why the plan itself is taxed, only that we don't get any of that tax money and it's largely dependent on local government and abiding by federal government standards. I know that's not a crystal clear answer by any means, but you can typically google each of your taxes in relation to your residence and figure out more!