The etymology of the term stems from early soccer rules (which early hockey copied) where forward passes (and in the earlier half of this early era, closer to the attacking goal than the ball/puck at all). A player in violation of this rule was said to be "off his side of the ball." Pluralizing it doesn't really make sense if you consider the origin of the term, but language does evolve, and that includes etymologically inaccurate words, such as "funguses" now being an acceptable pluralization of "fungus" instead of insisting on "fungi."
Then again, English doesn't actually have an official governing body for the language, unlike, say, metropolitan French (though L'Académie Française is a bunch of pretentious blowhards).