Because there may be police officers on the way down:
Armed police abseiled down a building to foil an armed robbery just as the thieves were breaking into a bank. The officers then set off stun grenades to distract the gang who were arrested as they tried to get into the HSBC in Bermondsey, south-east London. The men were found to be in possession of a taser, knives and plastic hand restraints. Three men, aged 26, 31 and 32, have been arrested on suspicion of conspiracy to rob.
All sounds a bit Lewis Collins in Who Dares Wins to us.
The photo to the right depicts some very different boys in blue, but they're on Borough High Street so we should get a B for effort.
Thank god the police wear pants.