Actually, I think there would only be a case if it could be proven that Norton was attempting to create a monopoly of some sort. Simply offering a product or service, which happens to put someone out of business, is not illegal. I imagine a lot of carriage manufacturers went out of business when cars came along.
Still, it's a shame that software can be provided on such a wide basis that blocks so much of the user's online experience without so much as their approval. I mean, do you know how many NIS users don't even realize they're missing something?