Okay let's not make a distinct rule so no non flying passengers in light aircraft can fly with a single pilot but maybe more awareness could made during PPL training about responsibilities towards your passengers. When I did my PPL wasn't mentioned once.
Not sure when and where you did your PPL training but certainly where I teach this is a regular part of our training - indeed Human Factors and Performance is part of the EASA syllabus.
Part of the individual pilot's responsibility is ensuring he/she is suitably licensed (including valid medical which is relevant in this context) and also fit in every way for the proposed flight.
Recently I had a student who has not yet soloed but did an unusually poor flight with me for his stage of training. This person runs a very successful world wide business. As we taxied in he remarked "I had a blazing row with one of my employees this morning". On the debrief I remarked that this was a good lesson for him - i.e. don't go flying if you're not in the right frame of mind - it could be fatal to do so.
A lot of this is down to risk management. If the flight is essentially "private" am not sure it is reasonable to apply the same legislation as you would apply to commercial operations except where not to do so might adversely affect others. It is indeed fortuitous in this accident that nobody on the ground was killed or seriously injured.