This is the official answer from their site:
No. Many Roku channels offer you high-quality entertainment for free. However, depending on which channels you want to watch, pay-per-view or subscription fees may apply. For example, if you’re new to Netflix and want access to thousands of movies and TV shows, you’ll need a Netflix unlimited plan, starting at $8.99/month. If you’re already a Netflix customer, however, you can start watching right away!
So it seems to be quite a flexible system.