Strong competition has a similar effect. If the SIM free contracts are good enough, people are not going to get into lengthy, expensive contracts. This doesn't happen in places like the US because there's a very strong cartel/oligopoly there that affords them the right to simply deny you a terminal if it's not under the carrier's conditions.
If Apple is selling for cheaper than the carriers can afford to sell, then they will simply do what they are doing: stop selling it.