As long as the powers that be want the OHL representative at the Memorial cup every year to ice the best possible team they can, every rule, or lack thereof, will be kept in place to ensure the formation of a super team or best team possible to represent the OHL at the Memorial cup. If you put a ton of restrictions on player/pick movement, teams won't be able to load up like they have been over the last 10 or 15 years and therefore, weaker teams will represent the OHL at the Memorial cup.
Also, the powers that be can also make the argument that it is better to send multiple picks, no matter how distant they are, back-and-forth in trades instead of players. Even though this league allows trades, it also like to have the least amount of player movement as possible so they can advertise that when competing with the NCAA for prospects.
One of the things that the NCAA brings up when trying to convince a player to report to their program is that the player will be in their for four years. The player knows he will not be traded to an undesirable destination.