You said the drums were good so I'm guessing all they didn't put new drums on. If all they did was grease the backing plate to make the constant squeak go away then that would mean that the shoe assembly is constantly moving and rubbing on the backing plate. And that would mean that the drum is out of round causing the shoe assembly to move. But wouldn't it have been out of round before the brake job and making a squeak then too? Take it back and complain again ... and again ... and again (especially in front of customers!) until they fix it correctly. Remember, the squeaky wheel gets the grease.