From AnimeCouncil²:

After establishing contact with extraterrestrial civilizations, it was found that humans were the only race to develop various sports. The number of alien sports fans grew dramatically, and the most fanatic of them regularly harass stadiums by taking part in a game, or by demanding a good show, etc. Finally, a treaty was settled, ruling all interferences towards other planets' cultures illegal. Rabbit Force, a group of rabbit-themed henshin girls, became responsible for enforcing the treaty and capturing any offenders.