I think the US doesn't care about anything going on inside Ukraine so long as they keep fighting the Russians. Prolonging that is all that they are interested in, now that they have understood that Russia isn't going to collapse and that Putin isn't going anywhere. If there is any fallout and instability in the region caused as a result of their arming and subsequently "betraying" violent extremists (which has happened before in other places and has always provided a convenient excuse to keep the US militarily involved), well that's just more opportunity to sell weapons and keep Europe scared and dependent on the US.