The US would have to physically provide proof of the launch location, word of mouth means nothing. Furthermore, this would have to be cross examined with other satellites from other countries. Moreover, the battlefield is always changing and the Ukrainians military and rebels are fighting up close and personal.
It should also be noted that Ukraine shot down a Russian airliner and denied it, so we know they are incompetent and they lie, we also have pictures of the Ukrainian military with Buk in the same area where the airliner was shot down.
The US has an agenda in Ukraine, of course they will claim it was fired from separatist territory, but that does not make it true. The US has been caught time after time lying about what is going on in Ukraine. Russia has been taking the blame in Ukraine for everything; likewise, everything that is coming from Russia is a lie, everything that is coming from Ukraine is true. All of the atrocities committed by the Ukrainian military and Ukrainian nationalists is simply ignored in the west.
When journalists confront the State Department about Ukrainian military targeting civilians the State Department goes into denial and even shamelessly praises Ukraine for "restraint" despite Ukraine using heavy artillery in civilian areas which resulted in hundreds of deaths, and a pattern of purposeful targeting of civilians. When confronted with a UN report about 110,000 Ukrainian refuges fleeing into Russia the State department claimed "we can't confirm that" basically denying facts. When confronted about Russian minorities being targeted the State Department goes into denial despite overwhelming video evidence not to mention there has been no condemnation for Ukrainian politicians and high ranking military generals calling for the extermination of Russians, calling for deporting east Ukrainians and putting them in concentration camps, threatening to murder civilians for every Ukrainians soldier killed, calling Russians derogatory names such as Moskol. Let me remind everyone this is coming from the Ukraine's most powerful leaders.
The pattern is obvious in Ukraine, if tomorrow Ukrainian soldiers execute 1000 innocent people and have it on camera it would not make news in the west, the west would simply ignore it like they did Odessa, Slavyansk, ect.