Doing something in public does not inherently invite being drive-by photographed. Just like if someone is dressed up for a dance club, they may want/expect to be seen and still not want to be photographed (and have said photographed shared) without consent. Legally it's allowed, but that's a separate question from politeness and morality.

I've never once had someone refuse, but it's part of a respectful interaction, and I have had people request the photo not be shared publicly online. Plus, if you ask them, they're likely to pose and show off the best angle of the costume.