What would those strict legal grounds be? Strict would imply you can cite something that references this?
Maryland DNR summarizes mooring placement rules here and by design or accident makes no mention of existing moorings: (www,dnr.state.md.us/boating/srmbuoys.asp)
Of course, there may be something in the actual regs that is not covered by the summary, but remember this is a hypothetical.
In the end, none of us can count on regulations, laws, or changes to them to really help out. The best that we can hope to do here is establish a sense of community values and hope that social pressures will do three things:
1) Limit the wanton placement of destination moorings
2) Establish who exactly the "Jackwad" is: the guy who places the destination mooring in crowded anchorages, or the guy who uses it without permission.
And, since it's rarely possible for the cruiser to know if a given mooring is either secure or a home base -
3) Encourage those placing moorings to do a really good job of marking them.
Wouldn't it be nice to see this?
"Private mooring 40ft-max. Use at your own risk. Leave your phone number if you go ashore"