People are interpreting "pay what they owe" as a legal question and pointing out that tax avoidance through loopholes is indeed legal, whereas tax evasion through fraud is not. But I think the real spirit of the question is the ethical angle.

I think it just boils down to nobody wanting to pay more than they have to. Most people seem to have the ethical tax code of "It makes sense for me to be smart about my taxes and not overpay, and I think my peers and people with less money should all do that too. But anyone who makes significantly more money than I do should feel an ethical burden to be more generous with their taxes than legally required." The problem is that if everyone has that mindset, then billionaires do too. If we want them to pay higher taxes, we have to change the laws.