And they've explained this multiple times. Aaron was given the title of co-founder, but he joined well after the site had launched, and then proceeded to go AWOL and not work for months until he was fired. Given that he was only very briefly involved in the site, it doesn't make sense to call him a co-founder, even if that was his de jure title.
E: At the same time, I feel like I should state - Aaron Swartz was a martyr, and I don't mean to disparage him or his legacy.
E: At the same time, I feel like I should state - Aaron Swartz was a martyr, and I don't mean to disparage him or his legacy.