Drupal GPL Questions
(Disclamer: As usual with legal questions, I am not a lawyer, only a lawyer can give you a definitive answer).

Every Drupal module that is written and every javascript code that directly interacts with Drupal (or any other GPL code, but note that jQuery is dual-licensed and also available under the MIT License, this is not relevant when used with Drupal, though), automatically falls under the GPL. That's why GPL is also called a viral license. Everything it "touches" is GPL'ed automatically.

If you give the source code to someone, then he has every right the GPL grants him, including giving the source code to anyone he wants to, modify it and so on. However, viewing a website alone does not distribute your PHP code. JavaScript is a tricky question, see for example https://softwareengineering.stackexchange.com/questions/125955/using-gpl-licensed-jquery-scripts-on-website-with-ads and http://www.gnu.org/philosophy/javascript-trap.html. As far as I understand these, you always distribute the JS code.

So, obfuscating the JavaScript without providing a way to get the unmodified source code or adding a statement like the one you suggested above is in fact a direct violation of the GPL and someone could attempt to sue you.

Also note that there is an extended version of the GPL, the Affero General Public License, which explicitly includes viewing of a website as distribution. CiviCRM for example uses that license, so if you are required to allow visitors access to your custom adjustments/extensions of your website, when using a software like that.

Given the way the Ajax system in Drupal works, most relevant things usually happen on the server side, so that is usually not a big problem. Also, most GPL related lawsuits are usually related to companies selling products which run GPL software (e.g. routers) which don't provide access to their customized source code. I've never heard of a lawsuit against a website violating the GPL, but that could just be ignorance on my side, don't count on that ;)