Retarded, in this context, is being used specifically as an insult by using a superseded medical term to imply that a person is of lesser intellect. The condition in question, intellectual disability as it is now known, is one that cannot be influenced by a person's actions, but is a consequence of birth.

In western culture it is usually considered offensive to use a characteristic that is a consequence of birth as an insult. For example: "Don't be such a black person/jew/asian" is considered offensive because you cannot control the trait of your race any more than you can control an intellectual disability.

Compounding that, as I mentioned above, the term 'retarded' or 'mentally retarded' is no longer used medically or legally, in the same way that 'moron' and 'idiot' aren't considered diagnoses anymore.

Therefore, using the term 'retarded' is culturally associated - exclusively - with insulting a person's actions by comparing them to someone who is disabled with the implication that a disabled person would necessarily act foolishly or irrationally.

It would be the same as if you needed an explanation simplified for you, and from then on every person who then needed a simpler explanation was then said to have 'needed a Tasuki'. You can surely understand, even if you personally don't mind, how that might cause offense.

After all, you are 'somewhat offended' by someone assuming that a link might provide a sufficient explanation instead of holding your hand through the explanation like someone who lacks reason, empathy, logic and intelligence ... Or do I need to Tasuki that further for you?