Why is it that some anime use language exclusive to the Yakuza? I was reading an article on language-learning hacks recently, and #13 of that list states (emphasis mine),


  Some popular anime (popular in America at least, and mostly reserved for little boys in Japan) uses the kind of language which is in the real world almost exclusively reserved for Yakuza.


Is this statement true? What are some examples of this, and why is it done?
The Introduction to Yakuza Japanese has a list of grammar, pronunciation, and vocabulary (and even lessons!).

There's also this Japanese Stack Exchange has a question: What are the stereotypical characteristics of yakuza speech?

And Japan Subculture's terminology page.

While Yakuza these days are mostly semi-legitimate, they still extort and is the largest criminal organization in the world. Just like how there's a taboo with tattoos in Japan because they're associated with Yakuza, the same connotations come with speaking like the way Yakuza do.