This is true, but as you just said: it has to be a core job duty made clear before hiring. I highly doubt that Yelp would tell candidates "you cannot have a sin

This is true, but as you just said: it has to be a core job duty made clear before hiring. I highly doubt that Yelp would tell candidates "you cannot have a single day without checking your email." There may be some implicit "this is a startup, so work hard" mentality, but that won't hold up in a court of law.