There's a copyright on their code, but there's nothing wrong with making something that functions in the same manner as other software, besides “under the hood” the code could be substantially different…
Now, disassembling code? That's another matter….
And if you think this is opening some hole for legal action, why aren't the makers of various linux softwares (for example) getting in trouble? I mean, making something free that works with MS Word, (even saves in its format!) and has many of its features, wouldn't that pose a “threat” to the word processing market? I mean, it even goes by the slogan “Star Office” as opposed to “Microsoft Office”… hmm…