Good point misterhorsey. I think the operative word in your post is "should". Unfortunately the airline, oil and other industries that would be effected by taxing the externality have more money and political clout than those advocating for your view.
Yes, its a point i do genuinely struggle with.
Here in Melbourne there was a drought a few years back. Residents were encouraged to conserve water with a target of 115 litres of day. However, this seemed to ignore the fact that the greatest consumers of water are industry and businesses. So diligent citizens who were showering with a bucket would be minimising their personal water usage, only to leave a greater reserve for the biggest users of this resource!
The comparison with carbon offsets isn't equivalent, but I see some parallels in the way individual behaviour can mitigate 'bad' behaviour by other participants.
I do think there are various ways one can effect policy change regarding global warming in a democracy: voting (vote green candidates), consumption (buy carbon offsets), behaviour (cycle don't drive).
But I'm still not quite convinced that buying carbon offsets is particularly effective change if it mitigates the pollution of others.