By default SharePoint 2010 renders its pages in IE8 Compatibility Mode. As IE8 does not support SVG this makes maps not work in IE9 which would otherwise display the maps fine.
Luckily there's an easy fix.
In the master file find the following line:
<meta http-equiv="X-UA-Compatible" content="IE=8" />
Now replace it with something like this:
<meta http-equiv="X-UA-Compatible" content="IE=Edge" />
The maps should be displayed fine now.