Abnormal behavior on the road increases risk by creating unknowns. I don't think we should ban autonomous cars from new behavior, but I think it's just as odd to say that unpredictable but legal behavior is somehow just as safe as predictable legal behavior.
But in this case we know that there are cases where a reasonable driver will be forced to do the behavior he is calling unexpected. Thus even though it is unexpected it is something that should be accounted for. If you cannot react in time then you need to adjust your behavior. I'm always amazed at how few people follow the 2 second following distance experts tell me is the minimum humans need for reaction time. (which works out to .2 seconds to realize there is a problem and 1.5 second to get your foot physically moved to the brake)
Legal != Safe.