design patterns - Why should text be non-selectable?

I've seen some websites that make their text unselectable. And I believe that this can break up user experience, as "selectable text" is a well established ui pattern on web and computer in general.

What is your opinion about disabling the functionality of selectable text? Is there any reason or example, proving that we should do that?

Edit: As was nicely mentioned in a comment, the above question seems to be targeted only for selectable text. But what I'd like to understand has mainly to do with cases where we should force text to be non selectable.

Answer

It's a myth that selectable text is "costless"

As a general principle you are right that text should be selectable. That said, since you're asking about non-selectable text, here are some cases to be aware of for disabling text selection.

There are visual elements containing text that users don't expect to be selectable. For example:

Let's walk through why:

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  Logos: some logos are based on text fonts and are even rendered as text. For most companies, it's important that logos are rendered faithfully so the idea of a user cutting and pasting half a logo text, or copying the text of a logo without the font, size, etc is unacceptable. This is a perfectly reasonable business decision for a company, so they may disallow selection on text logos.

  
  


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  Controls: it would be an odd experience for a user to be able to select text inside a pressable button or inside a dropdown menu. Sure, it may not do much harm but (a) it can confuse the intent of a widget; and (b) it's inconsistent with behavior found in most user interfaces so it breaks behavioral convention for users.

  
  


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  Draggable window titles. It's common for draggable windows to be grasped and dragged via the title bar. It would be weird if the user clicked and dragged a title only to find that she was selecting text instead of moving the window. For this reason, even static modal windows commonly have non-selectable titles.

  
  

These are just a few examples, but they illustrate a core principle that text selection can create negative user or business impact, and it can make good sense to disable it because it conflicts with primary interaction, impedes effective communication, violates marketing or business objectives, violates UX convention, etc.