Maybe somebody can clarify this for me. It seems that there is a difference in American and European laws about discrimination, where U.S. has a concept of protected groups (i.e. they explicitly enumerate who is not to be discriminated), while EU has laws based on equality of the groups (or more generally, humans). It seems to me that the American system is more antagonistic, in the sense it really prefers some group…
One underlying difference is that the US has at-will employment. Meaning an employer can terminate employee for any reason, except specific no-no reasons. In the US, it's perfectly legal to fire someone, just cause you think their haircut is stupid, they stole your girlfriend, they root for a sports team you don't like, or you just plain don't like them. I don't want to over-generalize, because Europe is a collection…
IIUC, correlation of any of those with any of the protected classes will land you in trouble though (and I would guess at least the second two from your list are definitely so correlated).
As a result, again IIUC, all the large companies(+) follow, in practice, a European-style approach: there has to be a well-documented, based on some clearly-non-discriminatory criteria (++), reason for firing you (hence performance-improvement-plans, etc, etc).
[+] large companies == worth suing - again IIUC small companies can basically do what you say
[++] I think that still ends up being easier than firing people in many European countries, though