Yeah, it's really the apparent cultural difference between metal and non-metal tours that piques my curiosity. It makes me wonder, what does one side know that the other doesn't? Are the early-booking/announcing metal bands ahead of the curve in terms of how to do this stuff, or behind the curve? I can't really think of anything negative that comes from an early-announcement; sure, the risk of (non-visa-related) cancellation grows proportionally with the length of the time window, and that can certainly be an issue for the reputation of an annual festival that people travel to (and obviously you've decided the benefits outweigh the risks for PPUSA), but I don't think cancellation of a normal tour risks much consumer wrath towards the band, and certainly not towards the other faceless parties involved. So it makes me wonder if the non-metal bands book just as early and simply keep the news under-wraps longer for some reason, or have they actually found a way to do the visa thing on shorter notice?
But as you point out, it may not even take a coordinated shift to make what appears to be a cultural difference between genres; it might just be the one(?) guy who books all these metal tours that has decided to do things differently. And not all international metal tours have had such big time windows. Enslaved/Alcest/Ghost was about 3.5 months. Devin Townsend/The Ocean was 2 months, with The Ocean being added only a few weeks before the start. Maybe those weren't Finberg productions? Or maybe they were just kept under-wraps better for whatever reason.
Anyway, I'm embarrassed to say that I have not kept tour announcement dates in my spreadsheet, so I can't make any graphs. But the bottom three shows on Reggie's (Chicago) calendar are Dark Funeral/Belphegor (2/8), English Dogs (2/16), and Deicide (3/10), with only one other show listed in 2012 so far. And Reggie's calendar is only about 25% metal. In contrast, I've seen 17 international non-metal bands this year, and the only one I can recall with a lead time over two months was Godspeed You! Black Emperor with a 5 month lead. The most extreme example in the other direction was the Chicago World Music Festival announcing its lineup a mere two weeks before the fest began! I'm pretty sure that was more due to organizational scrambling than anything else though, and even with such a short window, they still had two visa-related cancellation announcements inside that two-week period.
Neil