Bingowings said:

There is an even better explanation in the plot of the first film.

THE HOMING BEACON.

There would be very little point sticking one on the Falcon if the Empire intended to shoot and kill the escaping Rebels.

As Leia said, "It's the only explanation for the ease of our escape".

Why they are such lousy shots in the later films is where you need fancy theories.

That makes senses except for the stormtroopers in the sentry ships - would they have been briefed to attack and get blown up or not briefed and their ships handicapped in some way?