The only reason I can think of is that they did not feel it important enough to implement pseudo classes?? Or, more likely, they had already developed their own software to the point where changing it would cost too much, so they decided to continue as they were. Hopefully they will decide to become fully compliant in the (near) future...
If you are creating this ePub for yourself you can just use one of the better devices/apps to display...but if you are making it for others it is probably better to just put the <img> tag within the html document to make sure it gets displayed.
