All I can say is that people with a better understanding of the prosthetic/clamp involved and of the individual case deem this pilot safe to fly.
There are people on both sides of this camp that think they are right (as I am not discussing this with my wife I stand a chance of being right on this occasion!), myself included, but really all we can do is hypothesise because at the end of the day experts, I say again experts, have deemed the pilot involved safe to fly.
Another way I would like to describe risk assessment. Hands up anyone here who has flown with an ADD........yep me too. Who agrees that flying would be safer if we refused to fly with ADDs.........yep me too. Yet we sometimes might operate with one pack U/S (slightly mitigated by limiting our cruising FL), no APU, no autothrust, one generator inop........need I go on. We trust in the experts/manufacturers to come up with an MMEL and we then fly with the MEL. Every defect has been assessed for risk and as such has a 1 in .... number of becoming more than just a risk/threat. The point I am trying to make is that the risks associated with a pilot flying with a prosthetic arm have been assessed and found to be acceptable. If the paying public don't like it then that is fine, take a cab to Manchester or wherever it is you need to go but be warned, we all know about the statistics of deaths on the roads V dying in an aircraft accident!
I am too lazy and drunk right now to research what assessment takes place with regards to prosthetics, I would be interested to hear from someone who has overcome their disability to fly with a prosthetic, even if posted through a third party to ensure annonimity. I bet we would be surprised.