I think the difference is that you're seeing it as akin to someone saying "I was trying to work out what religion they were." Yes, it's illegal to discriminate

I think the difference is that you're seeing it as akin to someone saying "I was trying to work out what religion they were." Yes, it's illegal to discriminate based on religion, but in many parts of the country it's a big curiosity about other people (is he presbyterian?) and so many people may not be able to help privately trying to suss it out. The difference is that cast is a rigid hierarchy, based on society-accepted discrimination, and there is no reason to try and figure it out other than to decide where they are in the pecking order.