I can see why the RSPCA would prosecute this woman - if you own an animal, it is your responsibility to take care of it and be knowledgeable enough to do so. If you are feeding your cat substances that are toxic to it, you are not a fit cat owner.
I agree with MrsDeVere that people don't really understand what the RSPCA is meant to be for. I see it on forums all the time - someone posts about slightly less than ideal conditions an animal lives in, and an instant chorus of "Call the RSPCA!" appears. The RSPCA must have so much of their time wasted by people reporting things the RSPCA has no jurisdiction to do anything about. And then these people turn around and blast the RSPCA because, "they didn't care about the animal at all."
Perhaps there are instances in which the RSPCA really should have shaped up and acted better, but I think a lot of the flack is unfair.