The official stance of France and other allies in Africa is that they are in Mali on the request of Government of Mali to save them from terrorists. But I don't think this is true that you are spending the money of your tax payer to save a country who doesn't share even borders with you and even she is not in your continent. The actual reason is that France has large investment in Mali. As already mentioned that France fulfill its energy needs through nuclear power and these African countries are the large and cheap supplier of Nuclear fuel for France.
But when we see US is also taking keen interest in the whole situation then we are forced to suspect that it is also a part of some big game. It may be anti-Islam, anti-Al Qaeda.