Well, using the whole animal isn't really the controversial part. In fact that's more or less why ground meat exists.
However you can't just ground pure trimmings as that wouldn't be particularly nice to eat, you need some good meat in there as well. The 'pink slime' described here seems to be some process that separates out additional fat using centrifuges in order to make something that fits the official standards for ground beef without the need to add any good meat. It's highly debatable meat that's been that extensively processed should still be considered ground meat.
However you can't just ground pure trimmings as that wouldn't be particularly nice to eat, you need some good meat in there as well. The 'pink slime' described here seems to be some process that separates out additional fat using centrifuges in order to make something that fits the official standards for ground beef without the need to add any good meat. It's highly debatable meat that's been that extensively processed should still be considered ground meat.