But really, do we do any differently???
Think about it -- "weak" children are set aside from society into a room upstairs, or a care home, or some kind of environment where they are not exposed to the harshness of the real world.
The same goes for adults too.
Even if the spartans never did it, they probably put the weaker children and adults into a situation where they would never come into contact with battle or even slow a community for doing normal everyday duties.
Which is exactly the same as we do.
We put slower children into special classes. Put them on special buses, send them to special psychological and physical practitioners. For adults we put them into care homes. Or keep them at home and the government generally cares for them with special provisions.
And we accept that.
Putting infanticide aside -- are we any different from ancient cultures??