Lightroom 3 - Four Star Problem
open your Import dialog and look in the Apply during Import section at the right side: is there anything against the "Metadata" item, or just an empty box? If there is a Metadata preset showing, click on it and select Edit Presets. Select the same preset in the dialog that opens, and check if there is a rating being applied by this preset. If so, there's your answer: remove it and update the preset, or else stop using this preset for the next Import and that should be remembered for the following time.
If that was not the case, I can't think of other explanations except: the ratings were applied by some other editing or viewing or file transfer software that you have used before Import to LR, possibly without realising, were saved to the files, and then LR read these in - or, the ratings have been applied without realising inside LR, after Import, in one of the ways suggested here - or, another thought, the images are already present inside LR, with their ratings, and attempting to re-import them is simply doing nothing.