Shipping also very often uses the dirtiest of fuels and is therefore much more polluting than airliners which burn quite high quality refined fuels. That's why, to get around recent laws limiting sulfur emissions, large container boats now first "scrub" their exhaust gasses with sea water, so all sulfur will now nicely pollute and acidify our oceans. [0] No emissions though, because emissions bad.

I agree with you wholeheartedly that this is not a problem that should be solved individually. I don't hold my breath for what the higher ups are coming up with though, their measures seem misguided at best, deliberately loop-holed at worst.

[0] http://www.imo.org/en/MediaCentre/HotTopics/Pages/Sulphur-20...