Just adding my opinion as a ride operator myself. I work at a UK themepark aimed at 3-12s so granted my experience is a little different.
Firstly, I think the blame lies with both the ops and the man himself. Whilst it is a sad situation, I think it was a totally avoidable accident that could have occured anywhere. Our motto at work is 'if in doubt - check'. This covers us, as a ride op, and somewhat shifts the blame to the management team if something is to go wrong. If we are unsure whether to let someone on (ie most commonly in kids that borderline meet height restrictions), a phone call to a manager takes only a short while. Yes, this may slow down loading and may anger other guests waiting but in cases like this it's far better to be safe than sorry. There have been similar cases at my place of work and there have probably been many accidents that have been avoided because we used a combination of common sense, health and safety restrictions, and management approval. I don't know much about this incident but I would have thought that any doubt in the operator's mind should have been cause to have a manager present.
At my park we ask that all prosthetic limbs are removed as there is a risk to others' safety if they come loose and may hit someone if moving at speed (sounds ridiculous but this is straight from my employee's handbook). However, yes, there are probably times when limbs would go unnoticed eg if wearing trousers and normal footwear. If they have had the limbs for long enough, they may just walk with a slight limp. None of our rides (again, keep in mind this is a small park with two coasters that would be considered 'kiddy coasters' the height requirements are 90cm and 1m respectively if that gives you a better idea of the size of the rides) ask for a certain amount of limbs, however the bigger rides (the coasters, the log flume and rapids, the swinging ship) all ask for lower body control. This is specifically to brace oneself on drops/turns on most of the rides. I can't see how this man would have been able to brace himself against movements of this type, I think I read somewhere he fell during a sharp turn, I haven't seen the ride itself so cannot comment to deeply. In my mind, it is common sense that this would be the case. I also don't think that he would have been tall enough to ride without prosthetics, but that's a whole different story. In this respect the blame definitely lies with the ride team. However hearing that he did not listen/did not seek advice about suitable does leave him somewhat liable, in my opinion.
In short, I think we all agree there's no way the man should have been on this ride. Tragic though it is, I think that it was totally avoidable if the actions of both the park (the ride ops knowing and following procedure/ checking with management/ using common sense) and himself ( seeking advice/ knowing his limits/ common sense) were different.
