We hear "it's their platform, they can do what they want," a whole lot. The trouble is the platform is almost a monopoly on user created video. You can't launch on other platforms and hope to be as successful as being on YouTube. Their discovery mechanism and massive user base gives them an incredibly unique positions.

YouTube is from an earlier era, yet still after the dotcom bust. It was still on the rebound of the era of everything is free, we'll just make money on ads (like NetZero and Juno).

That model was and still isn't sustainable, and their dominance gives them pretty unique control over media. Same with Facebook.

Google made the option to mark a video as unmonetized in the first place, I don't see how it can be an uploader's fault for using an available option?

This analysis is incorrect. Google doesn’t require running ads inside videos at all.