Or, as is absolutely the case, because of the aggressive blocking and monitoring policies and restrictive usage policies in place in the kind of enterprises where IE remains common, they are exposed to the public internet and use public webpages, but with usage patterns which leave them underrepresented on the kind of sites that use third-party stat counters. OTOH, for certain other public websites, they are very large numbers of users.

IE can account for a fraction of total traffic, but it still can affect large percentage of users. The same person can use several devices (work, home, mobile) to browse the web at different times. I can't recount how many times I saw some page at work only to send myself the link to read it later. I'm not forced to use IE anywhere, but if I were, loosing that initial visit would also lower the number of visits from other browsers.

Yeah, you could just run IE6 inside Chrome/Firefox via wasm :)