I'm just guessing here but I think the reason Kicker actually didn't get a patent on a square sub/speaker, is because it might not be possible (hence why they patented the ridges on the edge of the surround instead).
You can't get a patent for anything/everything. There's certain things that can't be patented for a variety of reasons, often "legally determined", so my best guess would be that a speaker shape cannot be patented (at least not in the US) and that's why Kicker doesn't have a patent on it. (Could also be that someone else had the patent before Kicker and Kicker pays them royalties, but I never heard of that so I don't think that's the case).