Different balls for the same competition, why is it allowed?

We recently noticed, in Rugby, that different manufacturers balls are used in the same competition. In the RBS 6 Nations, Gilbert make the English, Irish, France and Scotland home games ball. Italy opt for the Mitre ball and Wales use the Webb Ellis ball. How strange it is for one competition to use so many different Rugby Balls. In this day of mass merchandising it usually follows that one supplier ties up a huge deal to secure the rights. Maybe one day this will happen for this tournament. Maybe the individual countries have huge deals in place of their own. Maybe, it's in the rules allowing each governing body the right use their own ball for home games. But how unique is it? If you know why it is allowed in this major competition, or if you know of any other competition, of this stature, with this oddity, let me know.