So you should be able to swarm Legions and builders just outside an enemy's borders and chop all of the forests and jungles and drain the marshes on tiles to which they will expand, before they can get to those tiles? Losing the reward for chops outside your borders may dis-incent that sort of "tactic," but it seems cleaner to just eliminate the ability to chop or drain outside your borders. Yes, it would be nice to be able to chop a forest or jungle here or there on your way to invade a neighbor (where there are no existing roads to speed your units' progress), but not at the price of enabling "gamey" tactics like mass chopping to deny your enemies various terrain features.