If anyone thinks this is hyperbole :
I worked for a tiny startup (>2 devs full time) where Oracle tried to extract money from us because we used MariaDB on AWS.
If you think this sounds ridiculous you probably got it right.
(Why? Because someone inexperienced with Oracle had filled out the form while downloading the mySQL client.)
Re-reading my comment in daylight I realize I got one detail almost exactly wrong: we were always <= 2 developers, but it seems everyone understood the point anyway - we were tiny, but not too tiny for Oracles licensing department.
Well... Serves you about right for choosing MySQL over PostgreSQL :)
Money. Anecdotally that's the primary reason Oracle do anything.