Math has a culture where author affiliations are often mentioned only as a footnote, at the end of the paper, and not at the front of the paper. For a field with such culture, where it has repeatedly been shown that quality work can come from many places, this kind of coarse gatekeeping creates a very unpleasant environment. I'm trying to imagine the motivation of the platform creators, but drawing a blank. Maybe it's a ham-handed attempt at spam filtering.