Maybe this is just my programmer mentality of making abstract connections between things, but is building on top of another software company's platform really all that new? What makes an operating system platform different than a web platform, other than the fact that OSes have been around much longer?

Any business needs to mitigate risks, and I'm not trying to downplay that. You can see this with Zynga and Facebook, for example. At the same time, you have to remember that the platform is also a business, too. Platforms don't have a business without customers to build on top of them.