Our government don't half pick an amazing time to introduce such a tax, airlines are struggling at the minute as it is and it sounds as though they want to make it harder for them to keep going by attempting to reduce the amount of SLF that travels with them.
It would seem to me that our government need to get their priorities in order... as someone has already said shipping creates more pollution than the aviation world but I haven't read of an oceanliner company pumping their own money into greener ship research.
The only reasoning behind it that I can see is that the majority of the public is ill-educated in this subject and think that air travel is the worlds biggest killer, so the government stamp another tax on travellers, leaving the general public thinking that they are actually making a difference.