Putting aside weather tear gas was appropriate to use at the border, this article in the Washington Post makes a false equivalence.
Tear gas is allowed for use in domestic riot control and SWAT raids because it is a “less lethal” alternative to lethal force. The point is to use something instead of bullets. Many “less lethal” tools used by law enforcement (tasers, flash-bang grenades, etc.) CAN be lethal, but are preferable to force designed to be lethal.
This doesn’t mean the “less lethal” options should be overused, of course. Excessive force by law enforcement is wrong regardless. But we should not confuse means designed to be “less lethal” in order to avoid lethal force with the use of chemical agents in warfare designed to increase lethality.