How come? Apple issues security updates for older desktop OSes regularly. Furthermore, apple does not install new mac releases (e.g. Sierra) automatically. You have to install the Upgrade macOS app from the App Store. Sure there is a big banner above the security update screen which advertises the new version but you really do not have to install it. And it certainly does not trick you into installing it (in macOS, iOS is different). It just makes it easy and obvious how to upgrade.