A writing prompt is an invitation to write.
It's a suggestion to get you started writing.
It's a diving platform to jump off into creating your world of words.
It's a starting line, but by no means a boundary.
It's a push to the hand and fingers to get going and write.
But it's also an idea or concept to work with and hence the need for it to be kosher. Going on the fact brought down by the Rambam (Maimonides)1 that we are what we think, and writing is thinking, the prompts here are connected to Jewish sources, to Torah and Mitzvot and ultimately to HaShem. Our purpose in life as Jews is to forge a relationship to Hashem and a G-d awareness in every aspect of our lives and of course, that includes our writing and story telling.
1) Moreh Nevuchim 3:51 from the book: Nefesh Shimshon Gates of Emunah The Principles of Faith Rav Shimshon Dovid Pincus, zt"l Feldheim Publishers, 2010