Why do they call it "freestyle" for a freestyle swimming

Photo by US Army, US Federation Government

Uploaded by Chris 73

My 9-yo son came back from swimming lesson at school, "Daddy I came 2nd in my group". I asked him what style did he swam. He told me they were freestyle, breaststroke, backstroke, and etc. Then he asked me back, "Daddy, why do they called it Freestyle?"

I told him that he had much to learn and that if he finished his milk, I would tell him...

Now, where is my wikipedia?