I'm sorry if this comes out as rude, but in Bitmixer's shoes, I'd do the same thing they did, and wouldn't feel guilty, or like I'm scamming you.
"Legally" (for whatever sense that word holds in the crypto-world) you donated 30btc to bitmixer.io. It was your own choice, and your personal mistake. God knows do I check my addresses thrice before sending, even after copy-pasting I can't help double-checking.
I don't buy that. They are a business. To stay in business you have to satisfy customers and maintain a good image, especially when it's something like BTC mixing where a lot can potentially go wrong and a whole bunch of trust needs to be earned.
If it was a burn address then that's one thing, but the guy made a mistake that's easily within their power to rectify and they seem to be doing nothing about it. Am I gonna do business with people like that? No, sir.
He shouldn't have done it. They should be doing something to help him out or at least give some decent reasons why not.