For one, outside countries don't get in the middle of internal/civil disputes. Only in extreme cases does an outside country intervene on behalf of people that aren't its citizens, when most/all the civilians are in favor of intervention, and only when the outside country will gain more than it could lose.

Two, mistreating enemy combatants will not only escalate the conflict, it will reduce the support of your allies. Mistreating your own people goes back to point one, so most people just don't get involved, and the citizens have no recourse.

Three, citizens have to obey the laws set down by their government. Trying to fight your government's abuses of its people is essentially going against your government, which will just get you punished.

Not every "riot" is peacefully gathering citizens. Most start that way, but often go far off the "peaceful" trail. That is when things like tear-gas are deployed.

Be careful using the plural there. The United States government is the only government from a Developed country that does this.

If you want to compare the United States to Developing and Undeveloped countries, go right ahead.