IMO it's better to describe it as an issue with Austin/Texas as a whole, instead of just tying everything to SXSW.

Austin & Texas have a lot of problems.

A volunteer asking for something unreasonable, possibly because he has no understanding for the religious role of a hijab, that you can simply decline, maybe explain and surely get sorted by talking to someone else, or at worst just walk away in disgust, is very different than experiencing police abuse and force, being helpless and restrained, isolated from your friends and family for hours.

Not to dismiss that event, still pretty fucked up, but not the same order of magnitude.

Fairness. Is that not sufficient reason?

If taking your hat off was required for the picture, then she should also have to take her hat off. If taking your hat off was optional, then she should be able to keep her hat on.