So, what happens in animals who swallow magnets is that on occasion, two magnets or a magnet and another object can be in different windy bits of the intestinal tract and stick together, pinching two otherwise unconnected parts of the intestine together, sometimes kinking them together sufficiently to cause a blockage, which doctors say could cause perforation of the intestine or other very bad things. So as many wil…
Some animals are purposely fed magnets to improve their health. For example, the Cow Magnet: http://en.wikipedia.org/wiki/Cow_magnet
Also, if a swallowed magnet hurt 0.1% of cows, but helped the rest because cows inadvertently eat lots of metal objects, then those 0.1% are seen as necessary collateral damage in the meat and dairy industry. On the other hand, if swallowed magnets hurt 0.1% (they don't, but bear with me) of kids, then it isn't seen as a necessary evil.