BUK is an old system which is relatively easy to operate for someone who had operated similar system AND is provided instructions by someone else, rebels probably have such people and maybe even defectors from ukrainian army who operated BUK before
however identification of targets is something which requires experience and I'm not at all surprised with this seeing ukrainian authorities letting civilian aircrafts almost cross paths with military ones...it was bound to happen and they are first responsible for this, they certainly knew prorussians had BUK system(the whole Internet knew actually), drunk russian rebels just fired rocket on what they thought was a military aircraft