Guess the club could say yes, it is your son's ticket but not anyone else's..............it's not transferable, so actually no, it isn't up to you.......as per the T&Cs you agreed to when you signed up for it........
It's a tough one - whatever the club do is going to piss some people off. Your son can't go, so his ticket (presumably) gets used by a pal of yours........meanwhile people on the waitlist for a ST can't get one (for example). There's no right/easy answer, but assume the main aims of the club are...
You can't pick a single restriction to prove a point. The club have to balance all the angles......or sell 25,000 'reserve your seat' season tickets and then let everyone upgrade depending on who is using it ?
Maybe they are trying to a) maximise revenue to reduce losses b) stop people taking the piss and c) trying different things to reduce the number of empty seats seen at every game (including sellouts)
Nit sure if any special circumstances apply in the case of illness. Suspect it would be considered.
Because people would presumably buy (very cheap) child STs simply to keep the seat. Imagine same principle applies for a full upgrade hence no longer going to be allowed now demand is outstripping supply (for a while, anyway).