If they want to write about other worlds and other peoples, they can. All they have to do is create a science fiction setting. If they want to write the ultimate love story, they can. It is all up to the writer. The moment they sit down at their computer or with pen and paper, they are allowed to write whatever they want.
Nonfiction is different.
It is based on facts, dates, places. Nonfiction is the who, what, and where of writing, while fiction usually deals with the whys behind the actions. Within nonfiction, there are many different types of writing from the essay, the interview, and everything in between. Both fiction and nonfiction have their place in a writer's life. I have often heard that a writer will not be able to make a living as a writer on writing fiction alone.