Seeing code should not be a security risk. Your code should not contain embedded passwords, etc. Also, the Open Source movement has shown that open and available code, combined with a mechanism for others to review and submit changes to that code, results in better security (in general).

The reason script source isn't available is rather one of these:

1. The source embodies a trade-secret or some sort of intellectual property.
2. Script-capable web servers are configured to execute that code, so no mechanism is in place to simply tranfer it -- putting the file up in a downloadable form is an extra step, and most people simply don't bother.
3. The code is either trivial or publicly available from a different forum (e.g. the Scoop code that drives Kuro5hin has its own site, so Kuro5hin doesn't publish it on the main site).