If it isn't, it's an elective expense - and you're choosing to add that expense on to the kids. And to yourself.
TMEA isn't required to do our jobs. Budget should pay for requirements.
As for convention - if your district pays for professional development, yes they should pay for convention. But if they aren't paying for the science teacher's convention, using your budget allotment to pay for TMEA is irresponsible too.