For me, it's just another example of the kinds of things Apple can get away with and no one bats an eye. 20% of all Mac users must upgrade or risk being unsecure. So what the upgrades are free? What if you have a critical piece of software that isn't compatible with the latest OS X? Or what if your hardware can't support an upgrade?
EDIT: Some people are making comparisons to MS dropping XP support, but I don't think that's a fair comparison since XP was released in 2001 and Snow Leopard in 2009.