A real world example would be an automated postal sorting/routing center with a bug that lets it be exploited as a remailer. By doing domain fronting, a single party is betting that this postal center is important enough not to get shut down/bombed. Obviously, the postal center isn't happy that you just put it at risk of getting shut down, because they deliver lots of other mail, none of which is obviously less important than your shenanigans. So instead, they opt to patch the bug that allows them to be unwittingly used as a remailer.

The core point being: Signal isn't using Amazon as a shield - it's using every single customer of Amazon as a shield.