I think the audiences of festivals are more often than not a classic hits audience. Sure there will be some fans who go to any festival to see Testament but there are also people there to see every other band and not Testament. It goes for all bands and I can see why playing a more classic hits package would work for such gigs. It's also one of the reasons I dislike festivals.
I see no excuse for them not playing a range of songs when they headline a gig and I agree that updating the set list keeps it fresh. I also think that given sales of music are supposedly down doing different sets would allow them a range of DVD's to sell, or even stream even though that doesn't make them money.
I do find it interesting that when Anthrax finally changed their tired set list they copped just as much crap about having the same set list, it's like people didn't even realise they'd changed. But then look at the promotional bonus JP got this past tour by playing songs that they haven't played for 20+ years. It can obviously be a good thing when bands do change it up, but I'm still going to blame festivals and such for the repetitive set lists and given that I'm sure I've seen more festival gigs from Anthrax and Testament I'm not going to be swayed