There's an expectation from the publishing/serving side of the equation that the content is being served to a proprietary app or a web browser that works a particular way. With the browser being one of Chrome, Firefox, etc., along with Google's YouTube apps.
On the user side, especially those who understand how the content is served, that the browser is not the only abstraction allowed. Google themselves run bots to scour the internet employing all sorts of tricks to access and index content. There's a fundamental way in which the http protocol works and its content served that is client agnostic. Everyone, Google Search most of all, have benefitted from this.
Making tools other than browsers illegal will fundamentally break the internet in my opinion.
It's not as clear as let's say access to a public park on private land. But the idea would carry weight that youtube may control the manner in which their publicaly accessable website may be accessed.
Let's say you can access via the YouTube app which does not require an account, and now you reverse engineer that app to bypass the app all together. It's a bit like cutting a hole in the fence around the playground.
I'm not a fan of what happened it's just clear to me why it happened.