Because their site was hacked and the email was sent through the protestwarrior.com mail server using the protestwarrior administrative login. I am a member of ProtestWarrior, and thus am in their email list. Check their site, both the HQ and the forums have been taken down.
Guess the GOP has been made aware of this.
Too bad they picked New York to have the convention in. When I first heard it, I thought it was a mistake. Maybe not. Seems like anywhere else in the country (except L.A.) would have been better than NY!