The Russians are in Syria at the invitation of the Syrian government; we are in Syria at the invitation of, well, us (which has always been okay to us).
If we are in the country illegally and we killed 150 people who are there legally, how did this not create an international stir?
Wagner Russians are covert/mercenary mostly former spetznaz. They go at the will of Putin and or his oligarchs in Russia.
That was highly humiliating for Russia, so they write it off as the cost of business. They don’t give a left chit about those guys, they are expendable.