And also a very stupid thing to do.
If someone phones or goes into the ticket office they have to deal with it there and then. They'll have times when they're very busy and have to bring in staff from other departments, and other times when they have staff sitting around doing nothing.
People ordering in advance gives them something to do during the quiet periods, and reduces the number of people who need to be dealt with in real time. It makes the ticket office more efficient.
So by deterring people from doing this, they're just making life more difficult for themselves.