MOCKED for starting interview in C#

My recruiter for this large company wouldn't tell me until the morning of the interview (had asked two weeks ago). He told me that they said any language would be fine. When it came time to start writing the code on the board in the first coding session, I started writing the code in C# and told them what it was. But in the middle of me describing my tree traversal code, they said "you know this position uses Java, right?" And I said yes, and informed them that others told me that it's the algorithmic core that is focused on and that they said any language used on the board is great. Then they told me I needed to write it in Java. Then I turned to change some of the code. As I'm explaining, I turn around, and they're looking at each other, laughing silently. Then we covered two more problems. After the interview, I go get some water. They're ahead of me, and I hear them mentioning C# and one of them rubbing their hand over their head and the other one laughing. I wish I was Eurasian so that I wouldn't have to deal with this malcontent and extreme pain like this. Why on Earth do we have it so unbearably difficult? Why on Earth am I being manipulated for things I can't change?