Believe it or not, but I have been on TV about half a dozen times. Of course it was always local TV. I never got the katakana treatment. Here's my thought on that.

If a foreigner is on TV as a foreigner, in other words, if this person's role on TV is to be "foreigner", he/she will get the katakana treatment to emphasize the fact that it is a foreigner speaking.

If a foreigner is on TV as a person, in other words, if this person is not taking the role of "forigner", s/he will not receive the katakana treatment. How many times have you seen katakana in serious shows or the news?

On those comedy/variety shows, the foreigners are there to entertain in the role of "foreigner", and therefore they get the katakana treatment.

It's a comic device.

I don't think it's so bad, and sometimes funny for effect, but the problem comes, as Maciamo pointed out, from the overwhelming portrayal of (black) foreigners as wild, crazy or idiots, and no counterbalencing portrayal of them just as people.