AC 91-57 is a set of guidelines but what the FAA is saying is that it simply
doesn't apply if you're flying commercially. Their contention is that all the rest of their
regulations (which are not guidelines) become applicable if you choose to fly commercially.
Nobody has quite figured out what law is actually broken.
FAA seems to be making the case that they have exclusive jurisdiction over all airspace
and that the absence of a regulation specifically allowing what you're doing in their airspace
is the same as a regulation prohibiting what you're doing in their airspace.
I have no idea how that would play out in court.
I also have been wondering about the definition of "Navigable airspace", as their
stated jurisdiction applies only to it, and I think I could make the case
that where we often fly (below treetop level) is not navigable airspace as the FAA knows
it, and is quite simply outside of their jurisdiction. New sUAS regs may clarify this.
FAA Regs covering tethered balloons and kites (among others)
101.13 (b) is interesting as it introduces the concept of "shielded operation" which
exempts you from all the rules in subsection (a). It's equivalent to what I'm talking about
with regard to flying below the tops of trees and structures, being outside of their