Printing SBB tickets online is convenient, but the tickets you get are quite restricted. They're only good for one day, they are only good for one person, and there's no refund or exchange. By contrast normal tickets are good for three months and are quite liberally refundable.
I think the reason online tickets are so restricted is they have no way to mark a ticket as used! Normally the conductor punches your ticket when it's used so you can't use it again another day. But with online tickets punching doesn't do any good; you could just print another one and use it the next day. In other words, online tickets are not printed on counterfeit-proof ticket stock. So to limit exposure the ticket is only good for one day.
They could have built an online system where the conductor's computer validates your ticket against a central database that's updated with what tickets are used, but that's awfully complicated. Limiting tickets works, too. There's still one way to game the system; you could travel twice in one day on the same route with the same ticket. But that's not terribly useful, so they probably don't worry about it.
We had order #398666 on 2005-09-02 and order #467833 on 2005-10-14. Assuming order numbers are serial, that's about 1650 tickets sold online a day.