That's like asking "where is it legal to drive 35 mph ?" The answer would be: "Any place that a lower speed limit than that hasn't been posted ".
So with md'ing, likewise, it's : "Any place where it's not disallowed ". Which would be too numerous for anyone to start listing.
So turn your question around and START with a place you have in mind. Then look up municipal codes (park rules or whatever) to see if a rule or law said "no md'ing ". If not, then it's not prohibited.
Did you have a particular place in mind?