Nothing technically stops you. But if the government can prove you have been using Signal, all of a sudden you can be in a lot of trouble. This could be used for political oppression. Plus, the fewer the number of countries allowing E2EE, the less incentive there is to make or distribute such software. As it becomes harder to find, most people will end up using sanctioned, backdoored software, which makes the few that don't stand out even more.