I took your comment of - "Surely if you're going to cycle on roads there should be some minimum fitness level before being allowed?" to mean that if you weren't fit enough you wouldn't be allowed, and how else would that be enforced without a fitness test?
People are selfish in all walks of life, many people drive selfishly, many people are selfish whilst on public transport or whilst cycling - yes it annoys me and it sucks but unfortunately that's just life, I don't see any real way of changing how people think (or in many cases, don't).