Whether it's unexpected or not depends on the user, you could as well say ctrl+v pasting in windows is unexpected if you don't know this shortcut. Middle click paste in linux is a standard feature this OS has always had
Well, we can have a long discussion about what is "better" or not, but in the end it's just a preference, and also a long-standing paradigm.
It's fine to dislikes it; everyone dislikes some features (e.g. personally I really dislike autoscroll). The thing is that it's not "unexpected" because it's been like this for >30 years, since the 80s. Nor is there anything wrong with it: it's just something you can like or dislike.
Or to put it in another way: if I wanted my Unix system to behave like Windows or macOS then I'd be using Windows or macOS.
It also closes tabs, and can allow scrolling. These things are in different contexts and don't interfere. You'd need to be in a text box to middle-click-paste. It won't mess up opening a link in a new tab.