Technically, I don't think the rules forbid specifying orders that don't make any sense. For example, in face-to-face gunboat play, often people are allowed to make orders like "Par S Mun->Lon" for the sake of sending limited messages. Of course, reasonable limits have to be placed on how many orders one can make, and how ridiculous your orders can get. You wouldn't want people just writing stuff like "Paris says hey, Germany, let's go get England, I'll help you land an army on Lon, if you help me leverage an attack on Liv" and just almost turning it into public press.
I think the limitations that WebDip places on the orders that you can make are simply a design choice to reduce order entry errors and to keep the drop down menus reasonably manageable. I guess some gunboaters that prefer more expressive orders might have a reasonable argument toward allowing nonsensical orders, but the menus might get a bit overwhelming/confusing to beginners and probably annoying to others.
Given the design choices made on this site, this behavior would seem to be unintentional. Other than in these multi-coast situations, this website doesn't allow you to order a unit to support a move from it's own territory.