I think the turban example is perfectly consistent with what the author said. You ask "how would rules such as the uniform rule for the police ever evolve?" The answer is: in much the same way! Under this author's understanding of that example, the rule under scrutiny would be: "Every policeman must wear a uniform cap." The Sikhs, wishing to wear dastaars, would petition the chief of police to be able to wear a dastaar instead. The chief of police (or whoever), would then think to herself, "Well, is it all that important that our police officers wear matching CAPS, or is the important part merely that they wear headgear that matches the rest of the outfit and contains a badge?" She would then conclude that the more appropriate rule, which should apply to /everyone/, regardless of his religion, should be that every police officer must wear a hat of the appropriate color and that the hat must contain a badge. A Sikh could wear a dastaar; an atheist could wear a baseball cap or a turban--or a dastaar. The point is that /anyone/, not just a Sikh, can wear /any/ hat he chooses, so long as it meets those standards that /are/ considered too important to change, such as matching color and a badge.
I think another example might be more clear. Back in the 1990s, there was a big hullaballoo about Native Americans out West using peyote, a hallucinogenic, as part of a religious ceremony. Using peyote, however, was illegal in the state of Oregon, so a few members of the Native American Church were fired from their jobs and denied unemployment benefits after they were found using peyote. The Native Americans argued that they should be exempt from this law due to their religious beliefs, and many agreed. I don't, and neither did the Supreme Court (check out Employment Division v. Smith [1990]). And here's the reason: if peyote is "not that bad," i.e., if it's fine for /some/ people to use it (here, the Native Americans), then it should be fine for /all/ people to use it. If peyote is actually /too dangerous/, if it really /must/ be outlawed, then it should be outlawed for /everyone./ What someone believes about peyote should have no bearing on his right to use it. (This is the same reason we wouldn't allow ritualized murder of children even if someone claimed it was central to his religion--it's simply too dangerous!) It's /discrimination based on religion/ (or lack thereof) to say that Tom, who believes that peyote helps him communicate with a higher power, should be able to use peyote, but that Bob, who simply feels that using peyote helps him relax and to paint, should not. Why should Tom's belief be worth more than Bob's simply because a higher power is involved? And what kind of law should involve litigating someone's religious beliefs? Imagine: Q: "Isn't it true that you don't REALLY believe that peyote helps you commune with God?" A: "No, I swear I believe! I pray every day!" Q: "But I have a witness who says he saw you at Catholic mass last week! And here's a Bible we found in your house!" What a horrible mess that would be!
To get back to the dastaar example, then, all the author is saying is this: If a Sikh should be able to wear a dastaar, then an atheist should be able to wear a dastaar on the job, too. In fact, an atheist should be able to wear any hat of the proper color with a badge on it. His beliefs or lack thereof should have no bearing on his right to do something that everyone else can do.