In Switzerland, for tickets that have no specific date on them, they need to be validated before travel. At a train station, there is a validator machine but in other places (e.g. bus stops), there isn't, so one of the accepted workarounds is to just write the date on it with a pen.
The mixed class carriage probably makes his argument easier, especially if second class is full (because the controller could then clearly see that second class is full).
Funny enough, with people standing in the aisles, it does make life harder for the controllers and - without surprise - it is possible to take that ride without being controlled. (Most train rides of at least half an hour will almost guarantee your being controlled). Of course, no one ever (or should ever) ride the train without a valid ticket just to play the probabilities of not being controlled - and I've had my train rides, even late at night with not many passengers, of about an hour on the main line without a control of my CHF 13 fare - but suffice to say I've heard that having the speedy-efficient open gate system in Australia would lead to massive (and near unenforceable) fare evasion.