If Immersion truely has the patent and Sony is infringing on it; Sony is liable. In the very begining, Sony could have negotiated; they just chose not to. Lately, MS has been loosing all their lawsuits, and it has cost them heavily. The probably looked at this case and realised how weak an argument they had.
However, I am sure when MS was negotiating a settlement, they veiwed the stock purchase as a strong profit making move. If they new they were going to lose, they could assume Sony would too.