If we're talking standards, rather than specifics, the pre threshold area is part of the movement area, and must me maintained as such. If it's condition deteriorates, and it becomes unfit for surface movement of aircraft, they would have to remark it with chevrons, or x's.
If we're talking specifics, rather than standards, lets take a step back to reality. We're talking about light aircraft here. Prior to there being a runway at this location there was a piece of dirt, which seemed to have enough surface integrity to support light aircraft. To throw 12 inches of pavement over that dirt, and say the first 600 ft is unfit is ridiculous.
Again, I'm in no way advocating 747's landing before displaced thresholds.
On a different note..
I found an interesting airport diagram in the UK AIP for Denham. AD 2-EGLD-1-3. It has two runways. In the section for 'local regulations' it states.......
"a public road adjacent to the aerodrome boundary crosses the approach to runway 24. Aircraft should not descend below the glidepath, nor touch down before the displaced threshold."
Now, this is a note for runway 24 ONLY, if this is infact regular law, why would it have to be stated as such? They don't quote and other 'standard regulations' in this section, nor on any other diagrams I looked at.
According to the locate authority, isn't this 'basic aviation knowledge'?? Why would it have to be listed with other irregularities??