Bug: Logging in clears out your shopping cart

I get the weekly emails of new designs, I often find 1 or 2 I like, and end up adding to my cart. When I hit checkout, I remember I should log in, since you have all my address details and my threadless points, but when I do, my shopping cart gets cleared out every time. Usually I quit, sometimes I go back and shop again for stuff, but I really wish Threadless worked like Amazon where stuff in your cart isn't lost when you log in at payment time.

I captured a short movie of this bug (and described it on my blog as well):
http://images.metafilter.com/threadle...

The first one shows adding to cart, logging in, then going to cart shows it is empty.
The second bit at the end shows a shirt in the cart, then logging in, and seeing the cart get emptied. I've noticed this bug for the last year or two and I think it'd result in a lot more sales if it was fixed.