Unfortunately, we live in a world governed by money as a motivator. While you might not be in it for the money, many people are, to a certain degree (you know, to make a living and to be able to afford a decent life). If companies are unwilling to pay anything remotely close to what researchers' time is worth, then they shouldn't wonder when people prefer to sell the exploits that they find to those who
do value their work appropriately.
And frankly, we shouldn't be giving companies a pass for being cheap because "reporting it responsibly" is the right thing to do. These companies are benefiting to a great degree by offloading vital security research onto unaffiliated and unknown third-parties. Your time, as well as the time of any other hacker or researcher, is valuable and needs to be compensated. I don't see why it's fair to any of us that we should have to work for free or for low pay-outs just because we might be doing the right thing. Same goes for any other career that is badly paid just because "they're helping people".