'How could so many senior police be so gullible?'
Well. That's an easy one!
Ever heard of Bramshill? It is the Police College for senior officers.
For those who are not wet enough already upon selection, it teaches them how to forget all those unimportant things such as public protection and replaces it with an agenda of how to make snowflakes happy!
(You need those sorts of tendencies yourself to get there in the first place, of course.)