A) GoDaddy has nurtured a lot of ill will over the years, whether due to poor customer service, a sloppy user experience, or their absurd corporate culture of using bikini babes and right wing politics to sell, of all things, domain name registration and hosting.
B) Internet users can (relatively easily) transfer domain names. It's something they can go do right now, without much fuss and without having to police their own day to day purchasing habits.
C) GoDaddy, unlike the other businesses on the SOPA supporters list, is not a purveyor of intellectual property with a legitimate interest in preventing counterfeiting.
D) GoDaddy, as a technology business and self-proclaimed "innovator", should know better than the others, who have the dubious defense of being ignorant of how this stuff works.