An alternative scenario:- an early solo pilot has screwed up and is low, slow and already has the airbrakes shut on the approach.
The pilot sees someone standing on or near the landing area and doesn't know whether the said person has even seen the approaching glider, or what he is going to do (duck or move and in what direction).
It's very very dangerous for both parties - the pilot could try turning or stretching an already overstretched glide even further with nasty results.
Alternatively he hits the spectator who is looking through his camera viewfinder and failed to appreciate how close the glider is.
Having people standing around the undershoot area on any airfield and especially a glider site is totally unacceptable.
And has Mr SU2112 (the original complainant) any previous form for stupidity for which he has already been reprimanded - and the reprimands have simply bounced off? The statement "I know what I did was wrong, but what was the big deal" seems a bit telling.
Or maybe the instructor had just been having a crap day, with his wife bitching about his having to go and instruct yet again - and all unpaid and voluntary.
I would really like to hear the instructor's version of this story before criticising.