Crossposting in a context of a platform rife with re-postings doesn't sound too appealing.
Repeat content in inevitable, of course. But on Reddit, it is accelerated because of the karma system, bots, and karma farming. The desire for more content and traffic in the fediverse is to improve the quality of experience for existing users and to make it more enticing to new ones joining. I would argue that is not the purpose of content generation on Reddit.
Maybe I'm muddying that aspect of the question, though.
They're moderated by whoever created them. I imagine if they permit something that is sufficiently egregious the instance creators/owners might delete the magazine? In the meantime, a user can block any particular magazine.
afa a magazine being take over. A similar process as with r/Canada and r/onguardforthee would happen: a new community could be formed. It could even the same name, just on another instance (e.g. canada@kbin.social vs canada@lemmy.ml)