Paid for volunteering?

I've been volunteering (on one level or another) with the USADSF for a little over two years now. Generally speaking there isn't any pay involved - that's how volunteering is supposed to go. The first time I volunteered I was still drawing my regular salary and I was volunteering at the place where I also worked - it was just in the summer when I was technically off. Fast forward to last summer when I "volunteered" again and got shipped off to Venezuela for 10 days (on 7 days notice, but I'm not bitter or anything). Now, that time I didn't have the option to continue drawing my salary because I didn't have the summer off, nor was I working at the place I was volunteering. My boss and my former boss (who orchestrates all this volunteer work) worked it out so my former boss covered my salary for me while I was away from my office taking "unpaid leave." Incidentally, this meant that I did draw my regular salary (sort of), but I did not receive any actual "pay" for my time volunteering. Are you lost yet? Good!
Now, on to the next volunteer opportunity. Got an email today that there will be some Team USA tryouts towards the end of the month at the place where I used to work, and they'd like for me to come and be the ATC servicing the tryouts. I'm happy to do this for the most part, and I'm accustomed to volunteering - at least for this organization. I then get an email asking what my hourly rate is so they can figure up what to pay me for doing this. Pay? I'm gonna get paid? Well this is pretty damn exciting! On the one hand I'm certainly not going to turn down any pay (especially considering that I still haven't been paid for the last time I did something at my old job), but at the same time I'm not sure I'm supposed to get paid for this supposed volunteer position I'm in. We'll be heading off to Taipei a year from now and I can assure you that I won't be paid for that time (but I've had 2 years notice on this trip and have had ample time to save up vacation days so last year's debacle will be avoided).
I guess it's not the organization who will be paying me but rather the facility. Maybe that makes sense. I've got no beef taking money from them if they need to hire someone to work this tryout. I'd just like to fully understand why, all of a sudden, I'm being paid to work with an organization I've always volunteered for.