Everyone entering a Restricted Area must be subject to the same strict security proceedures. I understand that the pilot is already in control of the aircraft and would therefore
probably not need a knife to take full control of the aircraft, but there is the possibility that he could have passed the weapon to another person who would then use it on a different flight.
A report by Rt Hon Sir John Wheeler highlighted that the Control Authorities (Customs, Immigration and Police) could be at risk of intimidation, coercion or bribery as they are not subject to search when they enter the restricted zones at UK airports. The same applies to anyone who receives a lower level of search than anyone else, this is why we need a uniform level of security.
The report can be found in pdf format
HERE (see sections 9.7 - 9.9 on page 9).