> Then why not speed on a track, or some other closed environment?

Because freedom.

> I imagine that shooting guns at targets is fun too - but most people wouldn't find that to be an acceptable excuse for shooting guns at targets in a public park.

What do you think the "we use them to hunt" justification is for the American love of owning guns? The overwhelming majority of people who trot out this justification aren't depending on a good hunt for any real productive purpose; they go hunting because it's fun.