That assumes there's use for a generator out near the pump. I don't think that's generally the case, meaning the methane would need to be stored and transported. Considering it's literally these companies' business to store and transport fuel, I'd assume if it were economically viable to do so, they would. That's not to say something like a carbon-tax wouldn't sway the economics toward that side, but it's not just as simple as "burn the fuel in a generator"