When I encounter a website which does not allow text selection, copying or right click, I usually enable the "Absolute Enable Right Click & Copy" browser extension which removes all of these restrictions.
Such restrictive practices, in my opinion, not only make the website less useful to the user. It also intentionally alienates its users.
I cannot think of a rational reason to do something like that.
Only the tinkerer-type techies. Most people don't understand why right click doesn't work, they don't have a mental model of what is responsible for what and things are often broken in mysterious ways anyway. If users are not alienated by how the web looks without an adblocker (try it once on some mainstream news site or blog or recipe site!), they surely won't be alienated by unselectable text.
The rational reason is to avoid getting their content "stolen", or having the user leave the site to do something else with the saved content.
Such restrictive practices, in my opinion, not only make the website less useful to the user. It also intentionally alienates its users.
I cannot think of a rational reason to do something like that.