If the only tool in your box is a missile armed interceptor, must every problem be a suspected hijack?
A missile armed interceptor isn't the only tool in the box of course, in this case it was a mere contingency for a non-UK carrier inbound to the UK with a very loosely defined threat situation on board. I'm not sure why people can't grasp this. We were an awfully, awfully long way from a shoot down of a civil airliner and the whole thing was relatively routine. The Typhoon was there just in case, what's wrong with that?
Jmmilner offers a good case as to why it's not that bad an idea to ensure that you don't routinely underestimate loosley defined threat situations.