Hi, I’m the rep for the plugin review team.
It’s not a violation as the plugin makes use of their external service. Services don’t have to be free.
Basically, the plugin operates in a similar fashion to Akismet. The comment is sent to their service, which analyzes it for spam and such, and returns a pass or fail. Same basic principle. Akismet is free, but only for personal use. It has pay options as well. The fact that this is pay-only doesn’t make it fall afoul of our guidelines. We care about the plugin code, and in this case, that code is indeed free and open. It just uses a service which is not free.
Keep in mind, people who make ‘services’ just so they can skirt this guideline will find themselves banned. The service has to be a REAL service, and not just some license check, nor can it offload essential parts of the code to create a false dependancy. We’re pretty picky on these things. YouTube is a service. Someone calling jQuery from their own servers to track users is not.