I assume whatever deal they had with Vudu, which for all we know was in the process of being done before the streaming service, is why those movies were only on for September.
That being said, while all streaming services at least from time to time remove stuff due to licensing and the like, there is a finite number of DC Movies out there of good quality that are not considered “so bad it’s good” at best, and the Batman movies, at least the first two were of the limited number of older ones that were considered quality movies. So I don’t think that having them up for that short of a time, especially in their initial month launched as a good thing. The advantage of streaming services is you can watch things at your leisure and while they did sufficiently let everyone one know there were not sticking around, unlike some of their comic books, I don’t think the amount of time something is up should be measured in weeks. You never get a second chance to make a first impression and that was not a good first impression.