Anything that is meant to be read as content should absolutely, without fail, be selectable and copyable (assuming appropriate permissions).
But stuff like tab headers, buttons, or even text-sparse tiles - things meant for the user to click on - can, and usually should, prevent text selection. It is super annoying to be clicking back and forth through tabs only to have some text erroneously highlight and then stay that way.
Exceptions to every rule, and to every exception of that rule, of course. But for the most part, allowing text highlighting in those clickable areas is a rough UX.
* note that I did not include anchor links; those are meant to be inline within text content and should therefore be selectable.