You mean, they have an obligation to do the work of writing a novel and give it away? They should work for free?
Bollocks. There is no contract between an author and readers to finish a series.
If you like a series, buy it when it is new, and get as many people you know to also buy it. If enough copies sell, the series gets continued. If a lot of books sell, then the publisher gives it more marketing dollars. The mill that created the book (i.e. the author) needs the raw materials (i.e. money) to complete a series.
Hard to write a book when your time is spent working your butt off in the local factory to feed your family.
I didn't say for free, did I?