What is the point of disabling the fullscreen button on a youtube video embed?

And funnily enough, I seem to find this on a lot of sites for software, that have a demo video embedded the page or some shit, like a screen recording in this tiny little frame where I can't read anything because it's in this 400 pixel wide box, that I can't fullscreen. I don't understand it at all! What purpose does it serve? You're actually encouraging me to leave your stupid site to view the damn video on youtube.com so I can actually read the text in your stupid ass video.

Why does youtube even give you the option to remove the fullscreen button in your embeds in the first place? They even recently removed some of the "modest branding" features, like hiding the title, or removing the recommended videos at the end, but they thought that this feature was valuable enough to keep?

This may seem irrational to complain about, but I'm confused and befuddled more than anything else? If I'm embedding a video on a website, the last thought I have in my mind is "Oh, I really don't want people to see my video fullscreen. Better make sure I disable that!"