Certainly you could cross if you had permission. One difference is that it was much harder to get that permission, especially for people on the Communist side t

Certainly you could cross if you had permission. One difference is that it was much harder to get that permission, especially for people on the Communist side to get permission to leave. (How many countries have you heard of today where that’s a thing?) Another difference is that the penalty for crossing illegally was often instant death, which tends not to be the case on most international borders today.