Lap bars require full legs to work properly and safely. That's just the physics behind what keeps you secure.
Each ride is different, each park is different. I believe there's generally no laws about this besides that the park must follow the manufacturer's guidelines, otherwise they'd be undoubtedly liable in the case of an accident. So the ride manufacturer will make these requirements for each ride. Sometimes parks strengthen them more.
The concern with prosthetics is that they could come loose and fly off, injuring someone or something. For this reason, most parks require prosthetics to be taken off for the ride, and then the normal body requirements are in effect from there. I know a few parks have started allowing prosthetics as long as they are certified by their manufacturer to withstand a certain level of g-forces, but I don't remember which parks. (Not Six Flags)
One thing Six Flags (and Great Adventure specifically) helped pilot is a restraint system for B&M coasters that allows more people with certain disabilities to be able to ride.
The tough part about this is avoiding discrimination lawsuits, so employees really can't approach a guest about this issue unless the employee is 110% sure they have a prosthetic. A lot of people sneak onto rides when they know they aren't allowed to be (most commonly with missing the height requirement), so the "I've been riding coasters all day" statement is very common, but completely meaningless.