I have been roofing for the past ten years, and I can tell you first hand that it is difficult to get rid of the shingles.  You can not simply put them in a garbage can, because there are far too many.  Almost every job that I have worked on requires that a large dumpster be rented so that the shingles can be dumped directly into it.  This way, the shingles are taken to a place where they can either be disposed of, or recycled.  Recycled shingles are the wave of the future, and we should try to do this whenever we can.