The problem I think is that Mustaine is trying to a)stay relevant to a new generation, and b)out OzzFest OzzFest. He sees that the only way (in his eyes) to stay competitive is to have GiganTour be somewhat like OzzFest. If you look at this lineup, this is an OzzFest-type lineup. CoB and In Flames have gone KoRn on everyone. JFAC is pure garbage. Who High on Fire is is anyone's guess.
It's just sad that for all the yakety-yak of not having a scene in the US, that instead of giving opening slots(aka 1 0r 2) to deserving American acts (Kat V, Suspyre, Zandelle, Stride, Outworld, I can continue for the next two weeks), they go get some cockrock band from a major label that does little of nothing for the headliners.
Let's be honest here: Who pays $50 for Megadeath tickets to see High on Fire?