You may have hit on how they could go about making money even after they open source webOS. "Here is the source. Do what you will; if you make money, expect to pay royalties under the following terms". If none of the 'old guard' licenses work for them, they could tailor a new license to their purpose.
I don't think that such an arrangement is in HP's character; however, letting the investment die seems foolish in any case.