This seems like an entirely reasonable design patent to me. There are lots of ways of drawing such a slider; the patent does not cover any function, merely the ornamental design.
Now, the idea that this should entitle Microsoft to all of Corel's profits for the entire product is clearly absurd; as the article points out, that's the current legal precedent but is being appealed. Lawyers are hardly going to not take advantage of precedents which favour them; nor would it even be good if they did -- the fastest way to overturn bad law is to apply it strictly and make obvious its failings.
Now, the idea that this should entitle Microsoft to all of Corel's profits for the entire product is clearly absurd; as the article points out, that's the current legal precedent but is being appealed. Lawyers are hardly going to not take advantage of precedents which favour them; nor would it even be good if they did -- the fastest way to overturn bad law is to apply it strictly and make obvious its failings.