It's chosen so that when the load is around 10 amps, the load that the 78xx sees is 1 amp.
With the 78xx flowing just 1amp, the 0.15ohm resistor is providing a voltage of 150mv: how is it going to open up the pnp?
I've been using this circuit for literally decades... believe me, it works.
I am sure it works. You may want to write the manufactures a note so they can rewrite the datasheet.