It's run like a franchise. When you book them they subcontract musicians local to you to go out and do the gig as "The Ship Band". There are several bands that work this way (see also http://www.licencetoceilidh.co.uk/), mostly aimed at weddings etc where the pay is high, but the audience is undiscerning enough to not notice that they have a scratch band. And of course, the band can be in several different places at once earning the agency more money on any given weekend. It's a clever system for someone to rake it in while sending others out to do the work.
I think 'fraud' is a bit strong, but maybe you're right to be indignant that the band you get might not contain any of the same members to "The Ship Band" that has played elsewhere.