Polemicists kept absurd anti-Islam tales because they were proprietary

Evidence

All writers tended - more or less — to cling to fantastic tales about Islam and its Prophet in a proprietary way, as belonging to the ‘Christian’ version. Polemists capable of highly informed argument inconsistently retained much that was quite untenable by our standards and that would have seemed ridiculous to Muslims. Those ‘facts’ which tended to show the falsity of Islam were preferred to all others. — Norman Daniel, Islam and the West, 1347