Imagine you run a relay that's not censoring speech a powerful government want to suppress. You're probably a small relay. It would be easy to shut you down with a DDOS, especially if you don't have proper rate limiting for every instance but even then, they can make it unusable with enough requests. Now imagine you just make every request cost 1 sat. For an attacker, they can pay the relay runner, or not attack. No other option. Legitimate requests won't mind paying a few sats and the info would be available everywhere and the attacker cannot afford to shut it down and if they do they make you rich so you can just spin up even more instances. Game theory.