Not necessarily true. If the field has a fence, a fence is a structure. If the CAA has a need it will bring all guns to bear.
Some years ago "beat-ups" became a bit of an issue with folk flying fast and low down a runway. The CAA "advised" for the purposes of Rule 5 that a runway is a structure and unless you were landing/taking off/overshooting from a genuine approach you could be treading on thin ice.