Yes, but you can tied the printer to a specific slicer in a number of ways. And you can make the electronic board in a way so that you cannot phisically update the firmware. (Putting it in a read only memory for example) You can alter the firmware (that you save is in a read only memory) to refuse to load gcode directly from a USB stick, you can have the firmware ask the slicer for a specific handshake protocol. Basically once you can couple the firmware with the slicer and make it not upgradable you can do whatever you want except maybe heavy cryptography. If the only way to change the firmware is to replace the board, I bet a lot of people would do not it and who would do it can simply build their printer from scratch. It would make the printer more exepnsive, sure, but that does not seems to be a problem to the law. Also, it would kill the opensource slicer (or at least try).