Hi Leah,
I have to disagree on this one.
Yes after registering the user is taken to MyAccount and given an opportunity to update non required profile information that wasn't captured during registration. The Newsletter tab is selected to encourage them to subscribe. At this point the user is on their profile page and has already registered but there is no state maintained to say this user just registered, the user is just on his profile page no different than if he clicked the my account link, the profile page doesn't know or care that he just registered, its only job is to provide a mechanism for the user to update their profile, it should not make assumptions that the user wants to go somewhere after updating the profile, thats taking control away from the user.
The same update button is below the tabs and applies to all tabs, a user can navigate the tabs and make changes and then save it all in one go (because tabs are just javscript showing and hiding different content already on the page its not navigation to different pages). However for the user this may not be obvious so they may feel compelled to click the update button on each tab and for this very reason we do not want to redirect anywhere when they do click the button. That would be an unexpected side effect and they may have really wanted to go to the next tab. When the user is done making changes to their profile other site content is just a link away, its not complicated. Especially the idea of text that says "Return to web site" seems a strange thing to say to someone who is on the web site already.
Best,
Joe