"Do you mind?"

I can't count the number of times I've heard the following exchange on TV:  "Do you mind if I ...(insert any action - "use the phone," "come inside," etc.)," and the prompt response from the asked party is "sure!"  Yet the asker ALWAYS goes on to execute said action despite having essentially just been told "yes, I do mind."  What gives?

It fair boggles the mind how so many different writers on so many different shows manage to screw up the English language as they write this exchange into their scripts.  I'm a freakin' engineer and I know it's incorrect.  It would comfort me if I could conclude that those Hollywood types are hacks, but it sneaks its way into NY shows as well.  Very creepy.  E.B. White would not be happy and neither am I, let me tell you.

Do you mind if I sign off now?  No?  Then I will.