I am sure that a club like Watford, who are in general, very fan and family orientated, would fully understand why someone is unable to go during periods of illness of difficult personal circumstances. And if these seats can be made available for others to take advantage of then all the better. They however, will be more aware of those who only miss the lesser ties leaving these seats empty. It will be these 'fans' that the club will look to turn away, if it ever came to that.
This isn't an attack on season ticket holders. This is a way to fill seats at games which would otherwise be significantly under capacity. Maybe it's me, but I'm struggling to see the problem here.
So the onus would be on the sick season ticket holder to prove they are sick to the club.
I don't have a problem with Watford trying to fill the stadium on less popular games, I have problems with how Watford are going about doing that.