In the case of
"Each couple is asked to contribute."
the word "is" applies to Each, not to couple.
So, the sentence would still contain "is" regardless of what each was referring to.
Each person is asked to contribute.
Each team is asked to contribute.
Each company is asked to contribute.
In the case of the sports teams, I'm guessing that the style was decided based on the fact that the majority of sports teams are named in the plural. Chicago Bears, New York Yankees, Houston Astros. Of coure it would be wrong to say, "The Houston Astros is playing tonight." So rather than differentiate the grammar between The Houston Astros and The Miami Heat, it just makes more sense to go ahead and consider them both to be plural.
One might think that the same could apply to musical groups. In the sentence, "Black Sabbath has released a new album", Black Sabbath is treated as a singular because it sounds like a singular. The same sentence, but with a different group, might not work. Obviously, you wouldn't say "The Beatles has released a new album".
A British gentleman I used to work with always used the plural verb when referring to companies. "Exxon are drilling in the Gulf" for example, when I would have said, "Exxon is drilling in the Gulf."