It works fine today. No work is needed by Google. They did more work to redirect the traffic.
Maybe Google is tired of the ongoing maintenance hassle and doesn't want to pay to write all the special-case code that will be necessary to support IE in future versions of their maps product.
From what I'm seeing, and to take just one example, IE 10 still doesn't support touch pan and zoom on mobile, which strikes me as a useful thing for a map application.
What work? They can just let Windows Phone users access the site and fend for themselves. I would say writing the code for the redirect is more work.
Except that the customers would then blame Google when something didn't work. No one likes to get blamed for the shortcomings of others.