If anyone could be a relay in a useful way, without permission.

Anyone can be a relay without permission. But no-one else will use their relay without in a useful way. Thus relays are centralized.

To fix this, among other things, nostr needs to find a decentralized spam control mechanism. This _can't_ be based directly on Lightning, as Lightning can't directly prove a sacrifice of funds because it can't prove that a payment actually happened.

You could use lightning for an indirect proof of sacrifice though, eg aggregating multiple parties in a merkle sum tree under a BTC proof of sacrifice.