Cost sharing an aircraft is not a question of safety, the rules are protectionist policy for commercial operators. The same as how car pooling was always a grey area to protect the taxi industry, since UBER who knows what the rules are... You could argue there are safety issues with a PPL selling under cost seats en-masse vs a regulated commercial operator, but the reasoning is more that there is not mass undercutting of the charter/RPT sector by pilots building hours or doing it for fun or tax breaks or whatever.