I'm not here over Christmas so my season ticket will be sat doing nothing for two home games. The club are not loosing any money. What happens in this scenario is that the cost per game increases for me or any other season ticket holder with any games any season ticket holder misses.
I can see where the club would lose money because if I or any other season ticket holder could not make a home game and had a friend or relative who was going who would be paying on the gate then they would be able to benefit from using my ticket instead of me. But the club are trying to encourage more people to attend and have been doing offers to get people in, so I see this as a recruitment drive by season ticket holders if they give their ticket to a friend or relative in their absence as long as it's not an upgrade.
If the club wanted to outlaw this kind of use of season tickets then they may as well just issue a card like the driving license photo-card and numerous other access and security cards that people have for work and educational facilities that have their photo on it.
Or have a bio-metric season ticket.