The problem there is with dogs, and has nothing to do with calories. Dogs (and many other animals) are simply not able to tolerate chocolate like humans can. Conversely, humans can't tolerate eating rancid meat and cat poop, but dogs can eat those things easily and not get sick. Lots of substances are poisonous to certain species, and non-poisonous to other species.

Also, the reason chocolate is unhealthy isn't because of the cacao plant, it's because of all the added sugar used to make it taste good, since raw cacao (or cocoa, which you get after cooking it) is horribly bitter.