Do haiku poems always have to be about a season or things in nature?

No, though it should refer to a season. It only needs to use a Word or phrase to indicate season, in order to paint a background image for everything else in the poem. It's mostly to indicate time and mood.

Here's a good article about it! http://www.associatedcontent.com/article/8361735/dancing_with_the_stars_haiku_poetry.html?cat=47