I'm not sure that's true... 99% of dwellings in, say, NYC are Condos, Coops or Rentals - and all buildings have management agents (or acting as your own as an individual for a building) on file with the NYC Department of Buildings.

Requiring a letter explicitly allowing a listing from the managing agent on record prior to publishing the listing wouldn't be particularly onerous. Of course their listings would drop dramatically since most aren't by the book, but feature not a bug?