Why do you think there is no licence problem? I could imagine that the problem is not the licence to display non drm mobi files but the licence to use the mobi reader. Maybe Amazon forbade Bookeen to use their mobi reading software in their newest devices. (It is such a cool feature that Gen3 and Opus can use a mobi and an epub firmware, I can't believe that Bookeen has abandoned this voluntarily).
Keep on reading in the free world!