Isnt polygamy being illegal an act of hypocracy?

You're mixing your arguments - you've accurately described the legal environment, but disagree with the existing legal conditions (presumably based on moral grounds). The legal world is full of contradictions; it doesn't change the fact that it's "the law." (Doesn't mean it's morally right, and/or (especially) that I agree with it! It just makes it the law at the time). Laws typically follow societal mores, and require change as society changes its outlook. I'd be willing to bet big money that, if society's viewpoint on polygamy changes, the laws will follow suit...