Because writing "young white men only need apply" is a fast track to a lawsuit.

Its obviously OK to implement it, plenty of examples of that, just not OK to talk about it in public.

What reason does the company have to think that young white men fit the job better? I can imagine that there are properties that these kind of people have that makes them a better fit, but why not advertize these properties instead? This should probably not lead to a lawsuite.

I think it mostly boils down to the "people like us" effect.

Because it's so easy to fake "cultural fit" during the relatively short hiring process.

I don't think so. For the reason why I disagree: Lots of cultural values take a lot of time to internalize, but are easy to check/hard to fake. For example if one property of "cultural fit" is "loves the UNIX way of thinking and writing programs" it is easy to check with few questions if he/she has internalized it or is faking it. Or if "cultural fit" implies "loves doing hackathons" the prospective employee will have taken lots of time at hackathons in the past. If there is nothing to be found in the internet of the hackathons that he/she was on (say, either from the Hackathon's website, pictures in social network, source code on Github/Bitbucket etc.), he/she is probably faking. etc.