A point & a question

The Point:-
When Ah waz a lad....
Jeans like these would have been either binned or patched LONG before they reached this stage of dilapidation.
I knew, fairly accurately, what denim was ... but ...

Bearing in mind
1. "Denim is a sturdy cotton warp-faced twill textile in which the weft passes under two or more warp threads. This twill weaving produces the familiar diagonal ribbing of the denim that distinguishes it from cotton duck."
2. "It is a characteristic of most indigo denim that only the warp threads are dyed, whereas the weft threads remain plain white."

The question:- Why do "they" need to tell "us" that the stuff's been dyed? As per "Extra photos"
 Do people imagine cotton is naturally blue?