In the wake of the Fastnet Race incident yesterday, I’m informed that there is already a similar provision in Maritime Law: UNCLOS Article 98 says inter-alia: “Every state shall require the master of a ship flying its flag… to proceed with all possible haste to the rescue of persons in distress”.
If at sea, why not on land?
While you have a point, I think the marine law requirement arises out of the peculiar characteristics of accidents at sea (ie. that the sea is vast and chances of alternative rescuers arriving slim). There are no readily-available emergency services at sea, so an onus is put on all vessels to provide such a service. That situation could arise on land also, but less often.
A Clatter of the Law 
In the wake of the Fastnet Race incident yesterday, I’m informed that there is already a similar provision in Maritime Law: UNCLOS Article 98 says inter-alia: “Every state shall require the master of a ship flying its flag… to proceed with all possible haste to the rescue of persons in distress”.
If at sea, why not on land?
While you have a point, I think the marine law requirement arises out of the peculiar characteristics of accidents at sea (ie. that the sea is vast and chances of alternative rescuers arriving slim). There are no readily-available emergency services at sea, so an onus is put on all vessels to provide such a service. That situation could arise on land also, but less often.