NATS is open source and CNCF. Redpanda is more a SaaS -- if it goes away, so does your app.
You're welcome to use it yourself and not pay us a dime. We're a business so we need to monetize but our core is to build a better Kafka. The nice thing is that if you've already invested in building against Kafka but you're exhausted by running Kafka, ours works better and faster. (Again, I'm biased so don't trust me, verify.)