Nearly all my friends who have PC's have Windows XP even on new machines. So unlike other browsers such as Chrome of Firefox, users can't update to IE9 or IE10. So in that sense IE doesn't support all the modern features PayDirt wants. If Microsoft allowed users to upgrade to IE9 and IE10 that would be a different story.

This company decided not to support IE. While your quick search showed no issues that doesn't mean there aren't or more importantly that there won't be in the future as they add new features. One would think why not allow all IE users to access the site and if something doesn't work oh well. But if they did that then all the IE users would email support anytime something didn't work as expected or look right. That's more time and money to deal with the users and the support requests.

Once again the big issue in my eyes is Microsoft requiring operating system versions (that have hardware requirements) based on what browser you have installed. For people using IE7 or IE8 and such they can install Chrome Frame however (no admin access needed) and that will add the necessary support and also assuming the developers added the necessary code to their site they can allow IE users with Chrome Frame installed. This line is included in HTML5Boilerplate.