Happens at most clubs I think. The Emirates was half empty when we went but still given as 60k. We have an exchange system where you can put your seat available online if you can't make it, and get a fiver off the price of your next season card every time you do this. I think Man Utd charge season ticket holders if they don't come?!
I went to the Vic to see a match there when Saracens used it as a home ground. It was dank, cold, and devoid of any charm. Fezheads I know are glad to be out of the place.
I'm sure most Watford FC fans love the place but if this 1 in 5 STH's who are allegedly staying away is true, I wouldn't be at all surprised if a number of them are fans who have decided that going shopping with the wife is preferable to sitting in an unattractive stadium watching an even less attractive team getting beaten.