Private ownership and voluntary exchange of the means of production is as important as free speech for the functioning of a free market.

This is how a libertarian would approach Nostr:

Property rights, a principle of non-aggression and voluntary exchange are considered to be the priority.

Relay operators can choose to reject notes from certain npubs because they have spent their time, energy and resources to set one up, which makes it their property. The only way a relay can be connected to is by voluntary aggrement. No coercion can be applied as Nostr doesn't allow that. You can set a relay up on your phone using citrine and orbot and use the outbox model to broadcast your notes, which people can make a choice about receiving.

Which brings us to clients. Clients are the property of developers as they have spent time, energy and resources to develop them. They can either charge for it or make it free. They can also choose to block certain people from using them. Nobody can force them to do so because Nostr prevents that.

Both relay operators and clients are well within their rights to charge people for using their services if they want to, as its their property and they have spent their time, energy and resources to build set up their services.

The Nostr protocol itself is a public good, as it is non-exclusionary and non-rivalrous. (Fiatjaf and other contributors deserve a lot of praise for keeping it this way.) Anyone can create new relays and clients and use the protocol in any way they see fit.

Clients and relays can make their source code open source, which deserves praise but their time, energy and resources are limited and they have to be compensated by someone for their effort.

What Nos and its users are doing is making a choice of using the protocol in a way they see fit. It's the property of the client developers. And users have voluntarily agreed to use it.

(This can be applied to Twitter as well. Elon and his shareholders are using their property in a way they see fit and people are using it voluntarily. TCP/IP and the internet stack however is a public good, for which the developers deserve a lot of praise.)