Fed misses lots of tournaments regardless, and more clay court oriented players don't have the pressure to attend hardcourt masters. Tsonga, Djokovic, Berdych, Del Potro all showed up as well because they can't afford to throw aways masters on their best surface.
No, they show up because this is a mandatory tournament.
Nadal skipped it citing injury. Federer skipped it because he's allowed to by the rules given his age and number of matchs on tour.
They'd get a 0 pointer if they withdrew with no valid reason.