The only reason they ever release things to open source is to try to get people to use their paid services. Eg: attention. They view open source only as an advertisement and nothing more.
The problem isn't that they use open source. It is that they spend a lot of dev times fixing all the bugs and then don't contribute them back upstream. They have sooo many internal patches that should go back but they go "if we contributed them back people wouldn't use our more reliable paid service based on that".
They are the exact opposite of the healthy hacker mentality that has brought Linux to the state it is today. Fuck Amazon * 100.
The world isn't a better place in any way with them. Their services aren't cheap in any sense.
Also, behind the scenes all those services you think are "ultra stable" are actual unstable and flaky as hell. Ask anyone who has worked at Amazon for more than a year and they'll tell you that it is constantly in pants on fire mode. All engineers are required to be on call because they have that many fires every single fucking day.