Spotters are not lambasted for fun.
Runway approach lights used to be mounted on stout telegraph poles. Boundary fences were also robust. Today we have frangible approach lights and the boundary fences at the thresholds are similarly weakened as landing short or over running, while rare, is always a risk.
Man in white van or on a ladder creates an additional hazard to life and limb should the aircraft have a mishap. It is not just directly on the threshold either but in a fan from the touchdown point.