Festival draw and tour draw are not the same thing though. I'd compare Grave Digger to a band like Falconer in terms of US draw. A Falconer headline tour would likely fail miserably but a Falconer PPUSA appearance would be pretty successful. The difference is that Grave Digger, at this point of their career, likely expect a headliner-sized paycheck, when they would absolutely not bring in a headliner-sized draw to the fest!
I'd also argue that most of those Nuclear Blast/Napalm Euro bands you're talking about toured the US before they had 'strong' album sales -- that is in opening/mid slots before touching the headliner ranking -- but that's another discussion for another thread and frankly one that's been had 10+ times now!!