It's mostly due to the fact that you install Developer Previews at your own risk. If you get any software issues with your Mac running Developer Software and you need to bring it to your local Apple Store, they will tell you to re-install Yosemite and install El Capitan on a different hard drive or anything that does not affect your daily driver.Why would? It's a computer, they can't void warranty because of software changes. Warranty covers hardware problems and to cause hardware problems with a software change it's pretty hard. Plus, you can always install the version of mac os that came with your computer if you want to activate the warranty and don't want for them to know that you changed the SO that came with the computer.