Well, I'd guess the pirates are just in their renifaire gear too

I mention the Houston games on our renfaire forum every year, and about every year someone asks about wearing their faire garb to the event. I tell them in as nice a way as possible that the games are not a costume event, to just wear their 'regular' clothes.
Plus at the Houston games faire garb is asking for heat stroke!

Look at it this way, most of us here have put a lot of time and money into having kilt related gear for different types of events, levels of formality etc.. Well, a lot of the rennies put just as much or more time and treasure into their garb. Just like some of us might wear Highlandwear outside of "traditional" settings, when many of them hear about a games or Celtic fest it just seems natural to dress up for it, with what they got.

Although I do wonder what some of the rennies, and indeed some of the "normal" patrons, were thinking with their getups, I'm glad they wanted to make the effort, and come out and enjoy the event. And I'd wager the organizers are happy they bought their tickets.