Every geek in the world thinks and says this whenever any legal proceeding involves inspecting hard drives. In the real world, it's just not that big a deal. There are chain-of-custody rules that apply any time digital evidence is handled in court. The reality is that the penalty for deliberately planting evidence would be so severe, the likelihood of success so uncertain, that no major corporation (or their zillion-dollar law firm) would risk it over a small-ball case like this.

(Like 'm0nastic points out, Sony can also pay a pittance relative to their legal fees to contract this out to Mandiant).