Just because they love al-Qaeda so much?
Never assume that just because a government does something, that they've taken into account all the available information and all the possible negative consequences. (Do you really think Obama has taken into account the potential for financial insolvency that could result from Obamacare)? I suspect that both Cameron and Sarkozy are hoping to accomplish some combination of the following: (1) appeal to the Muslim constituencies in their home countries (who despise the Communist Gaddafi), (2) ward off floods of Islamist refugees* if Gaddafi wins and (3) doing the fashionable thing by intervening in a conflict with no possible tangible benefit for either country (thereby following in Clinton's footsteps).
* I'd personally do the manly thing and send the refugees back instead of wasting fuel and ordnance.