Magazine Short Story Submissions

What are some possible reasons why a magazine publisher would insist on buying the copyright for a short story
submission? Of what value is it to them?
The reason a publisher would want to buy the rights to a short story is to guarantee they are
the only place that story can be found, and so they can make money from it in the form of reprints etc, rather than the author, who could send their story to every magazine in the country if they wanted to. Buying the copy write strips the author of their right to do that. I believe the author should consult a lawyer for guidance before agreeing, or signing on any dotted lines
Submission (Part 1) Written By Ayaan Hirsi & Directed By Theo van Gogh