capt. solipsist,
the same reason why we turn OFF APU Bleed just after engine start, or turn ON APU Bleed just before shutdown (after PARK BRK On):
That's to stop engine exhaust being ingested by the APU due to some air flow in that general direction at low speed / stopped. During the take off roll and flight - the APU inlet is in clean air - else they would have had to redesign it.