I speculated in a different reply that it’s to reduce dups. Two people could post the same article, but if they used different paywall-bypassers they would appear as separate submissions.
Link the original article with a paywall flag. If paywall, require paywall-bypasser, and link both articles to submission.
Link the original article with a paywall flag. If paywall, require paywall-bypasser, and link both articles to submission.