Why is tear gas banned in warfare?

Tear gas, or CS gas, is considered a chemical weapon that has been outlawed in war. But as a riot-control agent, it is legal to use by the police in the US. Here are the reasons why it is banned:

Chemical weapons are indiscriminate and “uniquely terrorising in their application,” which necessitated their ban in combat in 1993.

Once thrown into an area, chemical weapons cannot be controlled.

In battle, that would include civilians and even friendly soldiers.

An infant exposed to CS gas develops severe pneumonitis and needs to be hospitalised for a month.

US battle planners in First World War accounted for casualties from its own asphyxiating gas attacks..