So is the required overtime because there are positions which are unstaffed because there are not enough ATC's or is it to cover people going sick etc?
An earlier post said that someone gets called up one their days off to come into work. To me that sounds like someone has gone sick at short notice.
If you get called up to come to work on days off surely you can just say "no, I am busy". If you say "yes, I'll come to work" then that is your choice and you get paid accordingly. I'm sure some people would love the extra cash.
What's the big deal...just say "no thanks" and head off to the beach.