if she wants to buy a phone and plan and she has money to buy a phone and plan, then of course they should be able to sell it to her. It doesn't fall into the same category as the things you listed that are restricted to minors. Especially since t-mobile and most companies now just do month to month terms and not a contract (which a minor can't be held to).
Again I hope that this is just a bump in the road of life and that she doesn't get defined or forever affected by the dumb mistakes.