I think your take is right.
At best, it was a stupid teacher (sorry for the redundancy) who thought she could just waltz in and disrupt the business. At worst it was a union effort to embarrass Wal-Mart, which you mentioned.
I guess you missed this part,
According to Thelmas, the store manager gave her approval the week before to sing the one-minute song inside the Wal-Mart,
Sounds like they got an approval from the Manager, and some Assistant Manager wasn't told and took it upon him/herself to kick them out. If I was the manager I would be looking for a new assistant.