Remember, forcing you to watch ads means it's not free - it's you trading irreplaceable moments of your life for something. Unless your time is worth absolutely nothing, ads are extremely expensive to you.
Try running a hosting platform yourself and you'll quickly see just how crazy expensive bandwidth is. It's just ridiculous to expect that someone will 100% subsidize your video bandwidth for free.
This is begging the question that hosting non-monetized content is actually a net loss for YT. I'm saying it's actually not, instead the aggregate volume of free content keeps viewers on the site longer, and the higher viewership numbers result in a significant net profit.
More likely, YT decided that this arrangement, while profitable, wasn't making enough money for them, and because they must show their investors ever-increasing profit margins they decided to mildly poison their user experience in exchange for higher profits.
Google is in the process of breaking this deal themselves - they're now adding ads to videos which users did not opt to monetize, and they're keeping 100% of the revenue. Taking the content for free, if you will.