Im not 100% sure if this is right but I think your site uses javascript/server side code to check for a mobile device and adds the width meta tag with the width of the device. That may be why Bings not seeing the correct meta tag.
You'd normally set the meta tag as width = device-width which is in effect 100% and zoom level 1. But then I noticed your containing elements in the body tag also have a fixed width dependent on the devices width. Generally containing div's would be width=100%, that way there's no left right scrolling.
There's probably a few more adjustments needed somewhere I did notice the DOCTYPE is not html5 but html4
I quickly tried a couple of my sites and both passed with google and bing ![happy](/forum/images/emoticons/MLShappy.png)
Thanks for the heads up on that new Bing feature though, best test a few more...