And today they announced that Yoast SEO for Drupal 7 will be available later in September. Drupal has a smaller community overall but I bet one that will be willing to spend money at a greater rate than the WordPress crowd.
Yoast is collaborating on the project with the well-established Drupal agency, GoalGorilla. GoalGorilla has said on their announcement that the business model will follow the same freemium setup (paid upgrades, features, support) as the WordPress version.
WordPress-centric companies haven't always protected their IP, and some have paid for it. If I recall correctly, ManageWP's technology got ripped out of their plugin and used by competitors in times past. There's nothing wrong with that in GPL software, but if a company can protect their magic sauce, they probably should. There's a reason Automattic's most monetized plugins (VaultPress, Akismet) are just wrappers of proprietary tech.
I expect to see more companies make this kind of move, and I also expect Yoast will move beyond Drupal soon enough.