>why don't you just code a native app?
It sounds like that's exactly what they did,
They just hosted most of the code on a server. It's kind of like the "native" iOS apps that just use a webview component to display a web app. Sticking with IE6 just means you have all the hassle of a native app (like needing to make sure IE6 gets installed and stays installed on every machine) with none of the benefits.