I am not sure if I got Simon's question right (it is a little difficult to discern exactly what the question is) but if I did, you seem to misunderstand what he is asking.
He isn't asking why he can see the dot.
He is asking why he can see the dot the same regardless of what angle the light reflects off the surface and what angle he views it at.
Since the surface refects the light in a scattered pattern, his contention is that as the angle of approach, and the angle of the observer changes, so should the intensity of the reflected spot of light.
Is this right, Simon?
(See the poorly drawn attached image. By the way, that is a three-legged stool that guy on the right is standing on.)
It seems that Simon is saying that all 5 observers in this picture will see both reflected dots (assuming identical lasers) as not only having the same intensity and shape, but all 5 observers will see the dots as the same intensity and shape with respect to each other's point of view.
In other words, observer 1 will see two identical dots that are identical to the two dots that observers 2-5 will see.
What Simon is claiming is that since the dots are being reflected in many different directions by the intricate sand surface, each observer should have a different number of photons aimed directly at his/her eyes so each observer's impression of the intensity and shape of the dots (s)he sees should vary from the next observer's (of course, not taking into account physical differences in each observer, as if eack of those five positions were all specifically Simon himself).
Is that what you were trying to ask, Simon?
Click here for image
(edited to add the URL, because I couldn't see my attached image)