Yasmin Nair writes about the pernicious practice of writers being asked to write for free for outlets which should pay. I agree with most of her points, though I can think of a few instances where I profited in non-monetary ways from things I wrote and I'm still glad I did them. As a general rule though, professional writers should know the difference between jobs which can and ought to pay and the other kind.
It's just like any kind of work where the default is that people are paid for their labors…but there are occasional instances where it could be appropriate to donate. Usually, they are not when the project is being done for profit and the person hiring you is drawing down a salary. And they're not when someone is trying to launch an enterprise they hope will be profitable for them and they want you to work for free so as to minimize how much they have to invest to get their project off the ground.
I will have more to say about this in the next installment of our Rejected! series. Or maybe the one after.