According to Politico, the White House thinks pressure groups were signing people up for White House e-mails without their knowledge:
[W]hite House officials determined that advocacy groups on the right or left could have sent in the names without the person knowing it.
For instance, a group might have sent WhiteHouse.gov a comment from each person who had signed an online petition, and the White House would have captured the e-mail address.
A White House spokesman offered Politico a sort-of-apology for any confusion:
The White House e-mail list is made up of e-mail addresses obtained solely through the White House website. The White House doesn't purchase, upload or merge from any other list. … [A]ll e-mails come from the White House website as we have no interest in emailing anyone who does not want to receive an email. If an individual received the e-mail because someone else or a group signed them up or forwarded the email, we hope they were not too inconvenienced. Further, we suggest that they unsubscribe from the list by clicking the link at the bottom of the e-mail or tell whomever forwarded it to them not to forward such information anymore.
One reason it's politically stupid is that we live in a nation of imbeciles who don't understand technology and are desperate for more things to be angry about. Here's just one example of the many irate e-mails we received from wingnuts over this story—the writer acknowledges that she sent an e-mail to Obama, but is outraged that Obama shared her e-mail address with Axelrod:
[S]ince when does anyone email the president and get a "random" email from his senior advisor? ... I still say, okay, I understand I emailed the president, the problem is how did David Axelrod get my email address and why did he send me something I did not ask for? Who gave him my email address? Is he reading and answering the president's email? Did the president give him my email address? WHO in the WH is screwing up and passing email addresses around? I hope we get answers soon.