DISCLAIMER: I am not a lawyer, so do not take everything said here to be without fault. Please seek legal consult before making business decisions.
So once again, Nintendo shuts down more than 500 indie/fan games that were using their IPs:
This is of course perfectly within their legal right, and really based on trademark laws is actually required of them to maintain the mark, but a lot of fans and indie developers themselves seem to always cry foul when this happens. Many are often claiming that the work is "Fair-Use" or that because they've released it for free that it should be perfectly legal.