Some techs do it as a part of their regular job, for example someone may be a work at a hospital, and as part of their regular duties, check carseats. Or a police officer, firefighter, or professional EMS (Emergency Medical Service) worker. I think those are the most common professions where people are techs and get paid to do carseat checks as part of their regular job.
As a volunteer, I don't get compensated for my time or my gas to get to check events (about 45 minutes to an hour from me, depending on traffic). I think I can take something at least for my mileage when I itemize deductions on my taxes. I don't know if I can take anything for my time. I haven't looked into it yet. Either way, it will not add up to a whole lot.
I do enjoy working checks very much. Sometimes parents come in who are just there for the reduced-price seat. Sometimes they want their seats installed for them as an installation service, but aren't interested in learning. Those are the ones that aren't very rewarding. Sometimes parents come in because they truely can't afford a seat, but want to do the right thing for their child. Or they come in because they've heard the statistic that 4 out of 5 seats are installed incorrectly, and they've read the manual and really want to do the right thing, but they want to get checked. Those are the ones that are the most rewarding.