“incentivizes”?
The incentive is that you get to write events to the relay.
Discussion
Ah, you mean write events. That's different obviously. But one can just write events to multiple free relays. What's the advantage of paying to write events to any particular relay?
Every paid relay is currently “pay to write, free to read”.
Pay-to-write is the spam prevention mechanism, and the incentive is that you want a reliable host for your content as well as discoverability by others.
Hashcash (Bitcoin's Proof of Work) was invented as a pay-to-write model for email. Now we use it as pay-to-write on the timechain.