They could sell CDs with it on. They could sell T-shirts etc. They could sell fancy graphics / avatars etc.
The standard application from the Google Play store is limited in how many maps you can download and you can purchase the OsmAnd+ app for unlimited maps. Or... Get the same OsmAnd+ from f-droid :)
I quite like this model of charging users that are using the proprietary ecosystems. It sure worked on me, I happily paid for OsmAnd+ =)