"If you haven't taken an active, positive step to block our +1 buttons, we're going to assume you don't really care and we'll do whatever we can to show them to you, no matter what your browser's default settings are. Why? Well, because we think the default settings are bullshit, and 99 times out of 100 they're only that way because they're the default. They don't reflect actual user preferences, they reflect other browsers messing with our business plans."
Not only is that an intellectually honest position, it's a lot more accurate than assuming all IE users who haven't changed their settings don't want +1 buttons.