What does the fisherman mean by asking how?

See a joke (two fishermen are fishing by the sea. One day, a fisherman caught a mermaid. Above the fish's tail, she is a super beauty, but the fisherman wants to let her go. Another fisherman was puzzled and asked, "Why?" The first fisherman shrugged his shoulders and replied, "How come?" The second fisherman asked the first one why he wanted to let it go, and the first one said what if I didn't let it go! Because the mermaid's lower body is a fish, not a normal woman's lower body.