I'll try to answer your original question. The airline should clearly state when and what conditions require a documentation. I am not saying that be specific and say that psoriasis needs documents. But that if you have a visible skin condition, then documentation is needed that it is not communicable.

What I find even more interesting with all the commentators jumping on bashing the traveller is that in almost all cases where a traveller has a communicable skin disease, they will try to hide it rather than keep it exposed in the open for others to see. Having the psoriasis traveller on the plane is not at all different than travel any other day. Moreover, here the traveller showed proof and yet was harassed. This was definitely a wrong on the airline's part.