Except with most FOSS licenses, the application of the license has a direct and practical impact: You can hack on it yourself.

The problem is that Javascript is already like that (obfuscated or otherwise). I can step through even the crappiest JS obfuscation with a debugger and, say, find out if it's working against me. Or tracking me. Or whatever.

The license header stuff serves no purpose but to appease the paranoia of a subset of a microscopic subset of users who would not visit a website, on principle alone, because of the "freeness" of its javascript.

And this is not a slam against those users, it's a simple fact of numbers. Even with the most liberal possible estimates of how many of those users exist, and the most conservative estimate of what my theoretical engineers are getting paid, it doesn't make sense to bother with this.