The city of annapolis requires a permit. Almost anywhere else you are free to drop a mooring if it is not a danger to navigation.
I checked the MDDNR site and you are on point. Annapolis, Kent Country, Oxford and St Michaels have additional restrictions.
I think its likely the "general" restrictions that deter people from investing their money in a mooring.
"Single recreational moorings shall be placed in such a position that the area encompassed by the arc of the swing does not extend more than one-third the open water distance from the mean high water line on one shore to the mean high water line on the opposite shore. Also, the arc of the swing must not impede or obstruct access to the land of any riparian property owner, the access and proper use of any public access point, or otherwise hinder the orderly access to and use of waterways by the general public."
From what I've seen, the odds are pretty long that someone in MD (environmentalists, the rich guy on shore, the guy with the ski boat, the guy that crabs the creek you chose for your mooring, etc.) would take offense with your placement and you'd have to prove you met all those rather nebulous parameters. There is also the issue of how you get to your boat. You'll have to launch your dingy somewhere and that's going to be over private property and may prove impossible in many suitable moorage areas.
If the DNR published a map highlighting areas that met the requirements, you'd likely see a lot more people avail themselves of this possibility. I likely would as it could cut my ownership costs by more than 1/2 if the logistics were reasonable.