Ok, I got the point! I had forgotten that this “perspective” approach had slipped in at the final stage of the proposal, as this can prevent incorrect data in many cases - but can actually generate incorrect data in the case of a direction flip (a case that fortunately should only rarely occur, especially as the nodes are often located on oneways). And thus indeed stands in contrast to the use of side in other contexts (which is unfortunately not documented!).
Are you aware of which editors are capable of line direction flips and which of them perform them without asking users whether the new data is correct? afaik JOSM is asking, Vespucci is informing and iD is just doing.