Hmmm...pretty sure that Jedi are all for racial diversity. And, if you believed something (for instance, the dark side) to be detrimental in the long term to not only him who believed it but also to the universe, wouldn't you prefer people not to accept that as a way of life? Just saying. The idea that someone from the Dark Side could be accepted and turned is not completely without precedent - see Return of the Jedi.

Unless I was missing something, I thought that Jedi were always presented as defenders of individual liberty.

If you're talking about having or not having midichlorians, that's kind of a different question. Jedi partnered with all sorts of non-Jedi. But, you simply can't be a Jedi without some physiological predisposition, right? It's probably not a perfect analogy.