Hmmm, why don't they feed them grass? Isn't that what they are supposed to be eating? I'm no farmer, honest question.
Since grain (or in this case candy) has a much higher energy content than grass, cattle can be raised to market weight much faster than if they were fed a grass diet.
Edit: I accidentally a word.
Feed lots are very highly concentrated populations of cattle that are fed a regulated diet of grain (corn) mixed with concentrated antibiotics and, in this case, rotten chocolate. Another commenter already referenced Pollan's work, which exposes the tradeoffs of this industrialized approach. Among these are increased e.coli outbreaks (as e.coli concentrate in the rumen of the cow when it tries to digest grain,) and the increased likelihood of antibiotic-resistent bacteria developing because of the tremendous amount of antibiotics used to keep animals living in these conditions from getting sick:
http://www.oklahomafarmreport.com/wire/news/2010/07/media/00...
(That ankle-deep black stuff is poo, which also runs and collects in giant sewer pools that generate toxic runoff.)
The reason it's not done is cost. Grass fed beef is easily twice as expensive as corn-fed beef. The cheapest way to raise cattle is to raise them in a CAFO, and feed them the cheapest possible feed, which has been corn for quite a long time. The farmer who is feeding his cattle candy is really just feeding them yesterday's corn, as most American candy is made from high fructose corn syrup. Walk into a supermarket sometime and count the number of products that use HFCS -- it's staggering. The price of corn going up is actually really big news -- if it's a sustained trend it's going to have a far-reaching impact on American agriculture.
Without arguing about whether grass-fed beef is tastier in general, I wonder if anyone has ever done a blind test to discover if the grass (you know, the complex compounds flavoring the meat) can actually be tasted in the beef by an ordinary person.
As you observe, not everyone can afford grass-fed beef.
A lot of people say there is a taste difference, not just me. It's subtle though, your ordinary person may not notice or care. But I'm passionate about it because I'm big into food and I think the taste of grass fed is much better.
But I have never found a homemade burger to be tasteless, and I don't eat McDonald's any more for health reasons, so I wasn't sure what you meant.
I'm sure there would be problems such as trampling the grass, over-grazing, but maybe they're solvable??