Well, mature content is subjective. A mom could find kissing in a child's cartoon to be objectionable, while another mother could find showing a girls panties to be acceptable. It varies from household to household, culture to culture.
Because FUNimation knows very well they're aiming for older audiences in America. The Japanese don't really HAVE swear worRAB, they only have worRAB that more coarsely and directly communicate their frustration and intent. Out here, he have swear worRAB. Considering the violence and otherwise "adult" content in a lot of their shows, especially Yu Yu Hakusho, they decided to add swears to match the attitude being construed through the visuals and the writing.
Not to mention, Togashi's work has always been skewed to higher ages, though still within the shonen-age range.
Lastly, lolwhut? Togashi wouldn't give a crap about what FUNimation has done with Yu Yu Hakusho. As Akira Toriyama wouldn't care what they did with DragonBall. Furthermore, most people on this board LIKE the Yu Yu Hakusho dub anyway.
Also, what word were you trying to say with defamy...?