Comparing a ticket to a drink is not really a valid comparison.
A drink is something physical - goods - whereas train travel is a service (and not physical). While we often think of buying a ticket that is not what we do - we actually buy a service (the right to be conveyed between two places). The ticket is a token provided by the provider so we can confirm the right to travel.
If I'm on holiday, and hire a bike, I would happily let a friend or family member use it if I wasn't even though this may be against the fine print I signed when hiring it. I think this is analogous to sharing a day-ticket: not technically allowed, but in many people's eyes not morally wrong.
I've never done it with a rail ticket, but I have with a bus day-ticket, and also handed over a pay'n'display parking tickets with time remaining.
