I will note that being rerouted may not be under passenger control, so fining them could be a bit harsh. Meanwhile, passengers are instructed to bring allergies to the attention of the operating crew, as there's no mechanism to transfer allergy information around which is reliable enough (bearing in mind that for rerouting, this also has to work across all cross-ticketed airlines and when we see that ticketing systems can sometimes barely handle transferring names, safety critical info like allergies is a stretch).