Redistribution has independent lines of arguments, but insofar as the number of drowning children increase (or starving poor) then those who do not fulfill their duty that everyone recognizes as an obligation, they are free-riding off the duty of others. This is how enforceable action would arise: We all agree that we have minimal moral duties which have minimal infringements of our liberties (like wet shoes), so to avoid free-riders, we tax. In the world where there were no freeriders, no tax is necessary.