Nothing wrong with some fish psychology I guess, lets go on for a few more posts until it becomes too absurd.
Let's say a big fish for some reason decides not to eat the smaller one cleaning its mouth. Since I'm certain that fish aren't smart enough to figure out that the little fish' actions are good for it in the long run it has to be some sort of instinct telling it not to swallow. I also doubt that that instinct can be so specific so as to tell the predator not to close its mouth when a smaller one is buggering around in a certain manner in there, therefore the passiveness must be goverered by a more general instinct; perhaps something making it less aggressive, less prone to try to get nutrition from any source possible, less receptive -- in short, all explainations I can think of that are based on an instinct that would cause it to tolerate the smaller fish would make it less competitive in the ordinary race for survival.
And also if mouth infections where such a serious problem as to cause death, it would lead to the death of the fish less capable of handling such an infection (since the helpful behaviour of the smaller fishes which you talk of to come to exist on a larger, relevant, scale, would require an immense amount of time, whereas a new infection could arise comparatively quickly), hence making the entire species stronger and more resistant to said infections. In other words, mouth parasites are more likely a nuisance, not something that would cause a natural selction dependent on the job of the cleaning fishes.
Your argument regarding the small fish I can accept to some extent, though it seems an utterly unnatural foodsource and way of acting, something that would only be tried in extreme conditions under which animals would logically move somewhere else or simply die.