Google says you need to put your ads in because it assumes that you're going to put ads on a website that doesn't already have Google ads in place.
Even if you don't sign up for adsense, Hubpages will still have adsense ads on your hubs. Hubpages already set that up. However, if you want to make money, you need to sign up for adsense so that hubpages can easily share it's adsense revenue with you.
If you were to start your own website, that's when you would need to add some adsense ads yourself.