[Bullitt]

LJ: my profs in the past have also required proof of attendance such as a ticket stub, sometimes signed by a certain person at the event(s).

Fresh: It may not even be up to the TA. Notice how she says "we" instead of "I". It might be the prof's policy and the TA is merely the messenger. I feel your pain, I had some shit TA's back when I went to Ohio U who don't really give a rat's ass about you or your completion of the course. Contacting the prof was the right thing to do. See if you can set up an in person meeting or at least a phone call.
In addition, I would hold your tongue just a little bit because if you come off in attack mode to the prof, he/she will be much less open to working out a solution than if you stay cool and calm and reasonable. I would work the "Going to a concert is going to eat up valuable studying time because I intend to do my best on all my exams, including yours" angle instead of "It's not fair!", even though you're right the punishment for not doing the assignment doesn't fit failing the entire course.