It costs a lot of money to create a frame! You need skilled people to make one, get the proper archival glass to protect whatever you're displaying. There's a lot of work and field best practices that goes into this.
Are these optional? If not, I don't see how this makes sense:
>It doesn't really have to cost that much.
(b) They only pulled that stunt on art that was already behind suitably-protective covers. (Whether the stunt is effective or not, they weren't putting artwork at risk: just temporarily disrupting the operation of galleries, and getting themselves arrested.)
(c) This is completely off-topic.