"They write for the newspaper."
Translation:Ellos escriben para el periódico.
The simple answer is because it's para in this case. You can look up rules easily enough but i found you just pick it up and it seeems natural. You now know that in this sort of context (doing something FOR someone or some thing), you use PARA. Good luck
You use para because they are doing it for someone or for something where por is by someone or by something