Google has an incentive to be nice: they only make money and continue to exist if people use their products. If they suddenly take everyone's gmail address, the backlash would be incomprehensible. There would be Senate hearings and an infinite amount of Internet Hate.
But registrars are smaller and probably wouldn't face any financial consequences for stealing your domain name and selling it to someone with more money. You would be mad, but since it only affected you, there would be very little you could do. No hearings. No Internet outrage.
Alternatively, someone could sue you for your domain name, and you'd never be able to afford to mount a reasonable defense. Google, on the other hand, could afford to do that.
I think once a company gets to be Fortune 500 in size, they probably aren't going to do anything too drastic. It's the smaller ones that can be bought or do something unethical because they don't have shareholders or a board of directors. (Then again, that didn't stop Enron. But they fucked over their employees, not their customers.)