Airports, in order to qualify for FAA money (runway repaving, expanded taxiways, that sort of thing) need to have minimum standards. I don't know all the details as I am not an airport manager, but anybody providing a service for pay must meet the minimum standards at the airport they are operating out of. For instance, my FBO is required to maintain 1 mil of liability insurance for all work we do, be it instruction or maintenance (about $20K/year for our small 2 mechanic, 3 plane FBO). Freelance instructors and mechanics at our airport usually don't meet these requirements, and therefore are not in compliance with the "minimum standards". However, I don't know what the penalty would be for not meeting these standards. Just something to think about.

We often have customers ask why our maintenance rate is $15 to $20/hour higher than the local freelance mechanics. It's because the freelancers work out of the customer's hangar (no overhead), don't have liability insurance and don't use current maintenance manuals (OK if the FAA doesn't come around, and those manuals aren't cheap!). Plus, they only accept cash and maybe check, so there are no credit card fees to cover (about 3.5%, when all is said and done) and of course they never "forget" to report a cash payment on their taxes. They also don't need workers comp insurance or unemployment insurance, also not cheap.

Back to minimum standards, just ask the manager at the airport you are thinking of instructing at for a copy of the standards and they should be happy to give them to you.