I don't get it, either. Even the include is longer that directly writing a plain alert:

    <script type="text/javascript" src="warning.js" charset="utf-8"></script>

versus:

    <script>alert('You forgot to disable JavaScript!')</script>

To be fair, the following would be sufficient, but for some strange reasons the project recommends the long version:

    <script src="warning.js"></script>

that would be less funny and lack the educational aspect.