Why does an online shop need an specific browser?

I thought the times of stupid browser detection scripts had come to an end. But I was checking the new GAP's page, that was commented on 37 signal's "GAP backwards" entry and suddenly this page welcomed me:

gap backwards

The question is: why?

What do I need to browse an online shop catalogue, add some products to a shopping cart and, if I want, enter my credit card details and buy it?

You should be thinking: a browser!! Aahhh you innocent! For GAP people there must be something else that escapes my understanding, and it's absolutely needed to use their site. And for some weird reason, they decided that Safari is not enough for it.

Can't stop wondering what is in internet explorer which is not in Safari... Really...!