Says who, based on what data?

I feel like it's pretty clear that most educated people in the US of working age are either centrist or left of center politically, and that, accordingly, for the most part, groups of educated people in the US tend to have a "liberal bias".

I don't see what basis there is to argue that these companies should not provide a perk that a significant percentage of its workforce wants. I also think the term "bias" is misplaced; there is no duty on the part of these companies to resist culturally skewing as their employees skew. We can speak of biased media because, ideally, journalists should be as objective as possible, but that logic doesn't apply to, e.g., Amazon.