Too busy to get involved in petty stuff, I guess. If I had told them that I knew of a company with 5,000 workers using one copy of Photoshop they had illegally duplicated, that would get their attention since Photoshop provides a huge part of their income. But having people’s computers compromised by the inadvertent download of a fake update to already-free software is no big deal. Maybe sending out notifications to millions of users would be daunting. But acknowledging one of Adobe Reader’s users (that’s me) that he had correctly spotted some fakery would at least make him (that’s me) more likely to spot something more damaging that would be more worthwhile to the huge company. The point I’m making is to reward your friends with at least an acknowledgement of their efforts.
Unless, of course, you’re way bigger than any mere customer.