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Pretty sure that angle helps secure the basket or bag better. Though there is usually provision for securing underneath the bag or basket too.
Jon Blum
I have not used a front rack yet, but I have a question about the
design. Most of them have a tombstone that is angled so that it is not
vertical, but instead is angled like the head tube. Why not have it
vertical, since the bags used with these racks all seem to be vertical
rather than angled at the back? Just curious. April 26, 2017 at 7:35
pmJan Heine, Editor, Bicycle Quarterly
The main reason is probably aesthetics, but the angled backstop also
prevents the strap that goes over it from sliding upward.
https://www.renehersecycles.com/using-handlebar-bags-on-modern-bikepacking-bikes/
Am I asking for trouble if I try and bend the tombstone on an M18 forward so that it's perpendicular to the rack?