I'm going to respond here to something I saw in a parallel discussion elsewhere, which was a professional writer having a bit of a kick at hobby writers. Which, as a hobby writer, rather offended me. (And actually happened in person, once, as well)
Professional writers should not write for free unless for some very specific and particular reasons they choose to.
Hobby writers, in the vast majority of cases, are not competing with professional writers. Hobby writers fall into a number of categories, but broadly they produce works that are not commercially viable. This can be because they're not (yet) very good writers, or because the works don't fit very well into commercial forms, or they have a limited audience, or complicated rights issues. My 2000 word Star Cops fanfic (1) is not a threat to any published writer. Hobby writers are also, in the main, huge consumers of published writing.
I'm not entirely sure why it is, but writing, and particularly fiction writing, seems to be one of the only industries where some of the producers like to very publicly lay into the consumers for buying the "wrong" (i.e. not their) product, or not buying enough of it, or not consuming it in the "right" way.
TLDR: Professional writers shouldn't write for free. They also shouldn't tell hobbyists that they shouldn't write.
(1) I don't have a 2000 word Star Cops fanfic, but if anyone wants to write me one, I'd be very happy to receive it.