It has nothing to do with protecting the authorities, the authorities would do just fine with or without the checks. The purpose of the checks is to allow the service (scouts, schools, whatever) to continue functioning after a bunch of morons get in the papers saying "something MUST be done!" without fully thinking through the practicality and privacy issues of actually doing something. These checks are as far as you could go without being overly invasive and costing so much as to shut down the whole system - they do, however, nicely deal with responding to those whose words would otherwise close everything down.
The reality is that some people will always get into positions they shouldn't ideally get into. There is nothing practical that can be done to stop that aside from a weak deterrent. Our society is full of such weak deterrents - CCTV for instance, or the locks on your front door/windows at home. Neither stops the crime, they just deter the less determined from trying.