Having grown up with this being the routine, they probably haven't been very aware until recently that there are any places where it's not like this. To them, they're two separate events that some have now suggested combining, not a single event that somehow got split.
And keep in mind that the differences between white and black people in this country, especially in places that aren't very urban or affluent, aren't just physical. There are cultural/behavioral differences that make each much more likely to want to spend time around members of the same group than members of the other group. They don't talk the same, use the same body language, laugh at the same jokes, or have the same interests, and wouldn't choose to locate events at the same venues with the same decorations and the same food/drinks and the same music. For an event that's supposed to be fun, of course you're going to want to do it with people whose idea of fun is like yours. Two different groups with different tastes having separate gatherings that cater to the way each group would want them is the obvious natural thing to do. Trying to cram two such events into one is like trying to tell a gardening club and book club to have their meetings together, or trying to tell members of a Pakistani Muslim immigrant community and the nearest Lutheran church to hold joint services & events.
The law you quoted bans people from denying each other human rights and fundamental freedoms. It doesn't mention situations where those are not at stake. What right/freedom is being denied in this case? I don't think I've ever seen anyone making a case that being allowed to attend any and all social gatherings is such a right/freedom, but there are a lot, including those who wrote our laws, who would make a case that being allowed to choose whom to socialize with IS.
