Cameron believes, because his advisors tell him so, that psychology says behaviour only becomes extinguished if there is zero reward. Saving people from the sea rewards putting-to-sea behaviour, therefore, sadly, much as we would like to help, we must stand back and allow toddlers to drown.
Human social behaviour is far more complex than behavioural theory as understood by Conservatives can manage. Their psychology is doomed to fail, because migration behaviour is not in response to the pull of Britain's wondrously generous UK benefits system, but to the push from Syria's war.
Cameron is like the Leader of a Tory Council that has decided to cut the Fire Brigade on grounds that the presence of a fire brigade only encourages people to have house fires. He stands outside a burning house watching people waving frantically from upstairs windows. He is shouting "Don't worry, we're going to look at the causes of your fire".
We have to save the present cohort of refugees, and then take action to stem the flow at source. http://greenerblog.blogspot.co.uk/2015/08/how-can-we-address-causes-of-migration.html?m=0