"They let us go. It was the only reason for the ease of our escape."
As far as the Imperial Stormtroopers were concerned, the last order they received was 'let them escape but don't make it obvious.'
Tarkin gave the order soon after the Falcon landed in the Death Star. The Rebels escaped and blew him up. Nobody except Vader knew about it, since only he escaped when the Death Star went up.
Vader didn't tell anyone or rescind it because he wanted to capture Luke.
That was still the Stormtrooper's mission the whole way through ESB & ROTJ because nobody said to kill the rebels.