I also wonder who will be getting sued over this. If it was a Russian buk system would that make them liable.
War crime laws can be kind of weird, but I've never heard of a weapon manufacturer or the country it's located in being held responsible for acts performed by buyers.
It's not like this is a highly identifiable weapon. These launchers are all over the hemisphere - all the former Soviet nations and allies have them, China an most of its current and former allies have them, Russia and China have both gifted them to their pet dictators over the years, and they're available to buy on the arms market (and Russian weapons are still the go-to for most of the world because they're cheap. For the price of one Abrams, you could have two to four T-90 tanks instead).
As for the earlier quest about whether or not the rebels would know how to use it: First, a great many of the rebels are former Ukrainian military or former Soviet military. Second, they received some degree of support from Russia. Russia has acknowledged sending equipment, and acknowledged that people entered Ukraine from Russia to support rebels. Ukraine and many outsiders claim those are actual Russian military, Russia says they may be former Russian or Soviet military but are not currently enlisted.
So, yes, it's entirely reasonable and to some degree expected that they would have people able to operate a weapon used by the various armies they formerly served in.