So how come safely responding to a street hail is specifically tested in the
taxi driving test?

Because I was failing to distinguish between where they can ply their
trade and where they can passively allow themselves to be hailed.

No doubt their is some exemption that covers stopping at traffic
lights with their light on.

All of which pales into insignificance compared with that typo, for
which I am prepared to submit myself for the dealth penalty.