Indeed, the US should oppose those things inside their own boundaries where they have the right to. There is no reason to assume the US is virtuous and right on any given moral issue. In fact there are lots of reasons pointing towards the opposite case. We (I) don't want the US to invade other countries with its "concern" and cause mass civilian casualties.
So the US has a right to sell weapons to anyone abroad, but when it comes to the humanitarian consequences suddenly borders matter? If you want to be isolationist then be consistent. Having the benefits of international trade requires equivalent moral responsibility.
"There is no reason to assume the US is virtuous and right on any given moral issue."
There are lots of reasons pointing to the opposite, but that doesn't mean it can't be either. You could just as easily define 'right' as everyone intervening anywhere, regardless of borders, to prevent mass civilian casualties. One obvious case comes to mind.
The US hasn't intervened when it comes to mass civilian casualties in Pakistan, Kashmir, North Korea, etc, etc, etc. The US is hypocritical (like many countries) and intervenes when it is convenient.
On a related note - do you support Putin's invasion out of "concern" for the people of Donetsk?