Why is "is" used when there are multiple debaters?

Is can be used when representative of a group. Think of the group as one.

I think that the problem arises with the difference between American English and British English.

In the US, groups are considered singular whereas in Britain (and perhaps British-associated post colonial derivatives) groups are considered to be plural.

For example: in the US, one would say "Eastman Kodak is still producing film". In Britain: "Eastman Kodak are still producing film". - David Lyga