The "beacon/alerting system" are the signs, and the man in a vest. If the car can't respond to those, then the words "full self driving" should never be used to describe it.
But I don't really disagree. Some mechanism (perhaps a redundant mechanism) when approaching a possible construction area to give a human a minute or so to take over.
They just don't address use cases where people aren't able to drive at all. Which are interesting to a lot of people but they may just not happen for a long time.
There are people who just want cheaper Ubers but there are others who are fine with handing off long boring sections of highway driving.