The left hand does not always know what the right is doing. Here is one possible scenario :

Higher-ups pressure detectives to solve case. Detectives want to get promotions and kudos so fit up a couple of the often-reviled Burmese.....and conveniently get paid off by the rich family of the actual murderer.

Higher-ups know nothing of the set-up and a Basil Fawlty situation ensues, in which the detectives are locked in to continuing to justify their case....or else lose face in a REALLY big way.

Higher-ups continue to insist on police credibility.