I wonder about these tautologies myself. But, then, that always leads me to another theory/query: Perhaps the reason some people tend to add a redundant word after a common acronym, is because the acronym that was originally a noun, has now become an adjective. Not sure if that makes sense...let me try again.

The phrase Personal Identification Number is, two adjectives and a noun, as we all know, so adding that redundant noun is bothersome.

But, PIN the *word* has become commonplace as an adjective: You need a "number" to access your bank account, your email, etc? So what "kind" of number do you need? Why, a PIN Number, of course!

I think that is called colloquial speaking?