[stellar678]

I think it's mostly just the usability perspective, like was mentioned in the article. If you don't force the vertical scrollbar and this causes elements that should be in a static position to move, you've got a frustrating usability issue. (Like on sites that have forward/back buttons that jump around so you can't repeatedly click the same button. How frustrating.)

[Stormbringer]

That's not the usability perspective. Usability would be "don't mess with the semantics of the controls". E.g. if you're going to make something that looks like a button, it should do everything that buttons do.

Having the sidebar always on so as to avoid the bounce is not a usability thing. Usability would suggest that you leave the defaults the heck alone. In this case though, it is a pretty minor infraction.

Rather, it is an aesthetics thing. They've decided the default behaviour is 'ugly', and they've decided to override it to make it look better. In this case they've decided that prettiness trumps usability.