Its because the 9v supply is un-regulated.
Plus the other regulator is connected to a battery charger, and I had problems in the past using the same regulator for the signal line.
Seems like even after power goes out, for whatever reason the battery charger will leak backward voltage.
Plus the L78L05 is a TO-92 which is a small form factor which supplies the SIGNAL line.
Plus I have over 20 of the L78L05's laying around, lol
The other one is a LM7805 TO-220 for powering everything else.