The practice of exposing unwanted children was (almost) universal in the classical world. The sole exception, AFAIK, were the Jews. Ancient travel writers would comment on this extremely odd Jewish custom.
The Spartans were thus quite ordinary in their practice of exposing unwanted children. They were unusual only in that this determination was made by the State, not the male head of the household.
The Spartan practice wasn’t exposing, it was throwing the children to their deaths. One could argue that it was more merciful to implement immediate death. Of course, mandatory pederasty, fetishistic “marriage by capture”, the communal rearing (ahem) of children to ensure that they grow up twisted elitist killers takes a little of the shine off that.