Another interesting exploit; if you can't pillage/disband roads/improvements (or if the AI doesn't understand how to do so), then you could build a ton of them around the borders of another Civ, which will get socked with the maintenance penalty as their border expands onto the tiles.
Lots of problems like this once you start adding improvement maintenance costs.
Exploit is correct; and as the game is setup what you describe is possible. In SP I really couldn't care less and in MP I would quit the game immediately if my opponent tried doing this.
I wouldn't call it a problem unless the AI started doing it; in which case I'd complain that the AI is bad and not the game mechanic itself.
Its a game and I'd rather the developers focus on primary and normal gameplay elements and not worry about people figuring out obsecure exploits and attempting to prevent them.