It depends on where they work. We have to support IE6/7 because our users are largely accessing the webapp from hospitals... which means they're using computers that are very strictly controlled by the IT department, because upgrades can break other, very expensive legacy software that relies on old IE versions or other particular quirks of old OS versions. And if you break some essential medical software, lives can be at stake, so they can't take it lightly.
To be sure, they don't assume that our webapp will work in the browsers they're forced to use. But if it doesn't, we can't possibly get them as a customer until some distant day when they are ready to lay out the serious investment required to upgrade.