Maybe they're desperate for the true fan achievement.

I don't really see what the big issue is. If they want to support my opponent, so be it; but if I return the favor don't go crying! Plus, 2% possession really doesn't make much of a difference. So they're possession goes from 50% to 51%, if possession determines odds of victory it gives them a 2% better odds at winning. But since they're are multiple factors that determine how a team performs, a 2% possession bonus, increases their odds of victory by what, 0.5%?

When I have a friend in my league, i'll support them against teams that are abandoned or tanking but if both teams are in the race for a promotion spot, I don't support either side.