I never got the point of these awards, so they give out awards to whoever can make the best 5-10 minute demo.
Jeff or Brenden Sinclair answered this question a while ago on the Hotspot. The reason for these awards is so is that what companies can put on the box as winning an award of "Best of E3" is industry regulated an limited to a Grammy styled award system. This is a part of an ESA and industry led effort to "legitimize" claims made on game boxes to avoid false claim lawsuits. The awards and the members who vote are set up and selected by the ESA, which organizes E3, so the claim that the awards are "bought" probably don't have any merit.