I did not save the link to the article I read, but it said something to the effect that thyroid removal was not always necessary, because it was a "better safe than sorry" thing that made them always remove the thyroid if they found even one cancer cell. It said that in pathology reports after "cancerous" thyroids were removed, it was found that only that one nodule was cancerous, and a partial thyroidectomy would have sufficed, or something less drastic like RI treatment. I think they said that in the majority of cases, the cancer had not spread to the other lobe of the thyroid or to any other part of the body.
It's just easier to take it all out, and God forbid a doctor should have to do more than he/she wants to.