I've never given them my email address.
However, I did submit a complaint about their crappy service to the Washington State Attorney General using the online complaint form. Not surprisingly, part of the process includes the Consumer Protection Division forwarding a copy of the complaint to the company in question.
This complaint would, of course, contain my email address. So, I am guessing they pulled my email address from a service complaint and used it to send me spam.
This has the potential to get awfully damn recursive before it's over...