If requiring identification to the airport "third-party" is part of the terms agreed to by both parties prior to the sale, then I see no problem with it as no force is necessary and everything is done according to what you agreed to initially. Don't want to do that? Fine, don't agree to the sale and then consequently don't fly. That's how it would work if all these entities were fully free-market and not a semi-right granted by government.

Further, it's not "by force" if it's a pre-requisite for being on private property, which would be the proper analogy if you want to discuss this in an AnCap context.