Yeah I got that too and ignored it for a long time because it was for the Seattle tix that were originally Dec 23 but then I got a reminder after the rescheduled game saying I wouldn't get paid if I didn't complete the form (I just ask for a credit, too).
I though it was bogus because I'd sold other tickets on ticketmaster and never got an email like that but then I realized the email notification of the sale came from
ticketexchangesupport@email-ticketexchange.com whereas every other one came from
orderinfo@ticketmaster.com - so now I'm thinking they sold on the American ticketmaster website and that was why it was required.