Corporations actually do this, with respect to ideas for new products and product improvements, and it's rather smart of them. But their motives are different: They don't WANT those submissions or later claims of IP infringement, if they already had the same thing in the works.

To some extent, the same is true of traditional publishers, many of whom will not accept unsolicited and unagented manuscripts for this very reason. Not all "writers" are professional - some are, frankly, copyright trolls.

But Medium has been billing itself as a "blogging platform" and basically allows anyone to self-publish. The price of that should not be a wholesale surrender of rights that render "you own your copyright" meaningless drivel.