It's not preferred shares that are letting these founders retain control, it's having multiple classes of common stock where either the public investors get no votes for their shares, or the founders have 10+ for each of their shares, so that their vote always wins. This article gives a good overview of some of the changes that have occurred that have allowed this issue to proliferate (although the main issue seems to be a glut of capital and FOMO for these deals):

https://corpgov.law.harvard.edu/2017/05/26/snap-and-the-rise...