Google didn't 'make' the site owner change anything, there is obviously the choice not to serve ads and not change the site.

However, once the site owner decided to use Adsense then in order to use this service the site needed to change according to Google's requirements.

The point being made is that in order to serve ads the site owner had to add a lot of useless information irrelevant to what was driving traffic to the site in the first place.

Why did the first attempt get rejected, yet the final attempt after making the website objectively worse gets accepted?

The useless information that needed to be added to the site contributes to the decline in quality many people are noticing when using google search nowadays. This article provides a very interesting explanation for this decline.