I think this is a strange zero-sum way of interpreting things. It assumes tax income is fixed in some way, and that some not being taxed means others always get taxed more. That's false.
Why wouldn't it be allowed? If my investments into my Roth IRA performed so we'll i became a billionaire, I wouldn't want to be treated differently. And I wouldn't want my mom or my neighbor to either. They essentially got lucky, they're not taking anything from me in being so fortunate.
The "problem" in this case is that Thiel was already a billionaire to begin with, and such luck on top of existing wealth is what irrationally bothers people.