Who pays for a flight when refused entry?

I'm curious to know, who exactly foots the bill for a flight when someone is denied entry at their destination? Does the airline automatically absorb the cost, or is it the passenger's responsibility to find an alternative route or pay the penalty? Are there any regulations or policies in place that govern this situation, especially when it comes to international travel? It's an interesting question that I'm sure many travelers would like to know the answer to.