Are you Sure the brakes aren't still holding it on. Just because it turns freely doesn't mean the brake shoes arent' preventing it from coming off. Brake shoes ride in the middle section of the brake drum and over time will create a lip on either side of them where they wear down the material of the drum. Once the brake shoes get secured behind this lip the brake drum will not come off until you take a brake spoon and release the tension on them. on the backing plate there is a little hole take the brake spoon or a flat blade screwdriver and spin the star wheel on the self adjuster until it spins no more. you will know the correct direction quickly. if the brakes lock up you went the wrong way. I'll be honest with you I sale auto parts and have several ASE certifications. One of them being brakes--the first one I got. it has been my experience that most rusted on drums or drums that just won't come off are beacuse the shoes are behind the lip preventing them from coming off.