It is indeed customary because you are booking a tour up to a year in advance easily in this day. Until just very recently, the earliest you could submit your visas was 6 months before the day of the first show. That is why "premium processing" was instigated. You can get your visas done in three weeks for an extra $1,000. That's not a guarantee of approval, just that you will know something quicker. Considering they have until 90 days to notify you and then the extra 5 days- 3 months to get an interview, you run out of time very quickly. That's what the new "law" in the Senate is addressing. If you waited on your visa to be approved via the normal timelline before announcing a tour, you could not do it more than 2-3 months in advance.
That money is not much to a major artist doing a tour. Hell, that's not even that much in the grand scheme of things for a band doing a club tour here. Furthermore, in those situations, you can apply for your visa to be good for up to a year (or even longer) . In this situation, the booking agent or record label is the petitioner for the dates in the U.S.
Contrast that with a promoter doing a single one-off show for multiple bands that have no intention to tour nor do they have record label support here for the most part. I'm responsible for the band while they are here in the States so the visa is only good for a week around the festival.
The little guy is the one that suffers the most in this process.
Honestly, if I could actually speak to someone about the "why" of a particular case, that would make a world of difference.
And as an FYI....you have to do the exact same process for the crew seperately. They cannot be filed with the band. It's quite the moneymaker.