Riots do happen, after football games, unpopular court decisions or perceived
incidents of police brutality. Looting can start, or innocent people can wander
into the area and be in endangered. The police then have the problem of
maintaining order. They do not wish to shoot people, of course, and locking
up everyone involved would be complicated and impair their ability to police
the rest of their territory. In such circumstances, the police might justly decide
to use tear gas or something similar to disperse the crowd. Conditions could
make it the best of their bad options.
Tear gas should not be used on civilian populations. Tear gas is a cruel and unusual punishment that is barred by the bill of rights. It is disgusting that is even being considered to be use on civilian populations, because it is just not suitable for civilians to be subject to that pain.