Most (all?) major programing languages have easy to use libraries for openID & Oauth, both of which underlie this service. There is also widely-available browser-side code for users to pick an auth provider.
And, in the end, you still have to keep track of your users somehow, so it's not like you can actually get away with zero server-side code for anything but the most basic app.
That said, being able to login to the AWS console using federated credentials is a very good thing as it allows for centralization of auth credentials (makes it easier to change/revoke all at once...).