Movie rental curiosity

Hi, Over the past few months, I’ve run across an interesting business practice. I’ll start watching a movie that’s free, and when I go back to finish watching the movie.....it says I have to rent/buy the movie and it’s no longer free! This has happened 3 times in the last couple months. Is there some sort of policy that says the “free” movie must be watched in it's entirety? Or is this a business practice where the SOP is a little like extortion? Or do I just have the worst luck in free or included movie viewing as a Comcast customer?