There are two reasons why this is a problem.
It is not legal to distribute Ninja Shadowbox as-is. The GPL license only covers the code... PHP and anything it processes serverside. We still reserve rights to the images and CSS, which are not processed by Joomla and are sent to the browser independently. This is compliant with the GPL license. Replace all NinjaForge images and CSS used in it, and you can *legally* do what you want with it.
Separate from the legal issues...
I'm sorry but we do not allow any distribution of our extensions. You are legally allowed to distribute Ninja Shadowbox (provided you also follow 1. above) but it is against NinjaForge rules and will get you banned from the club.
We're not assholes (most of the time :P ) and we didn't create this rule without a good reason.
You see the NinjaForge club depends on its members to continue existing. We are able to develop new extensions and maintain and support existing ones due to our kind members supporting us with their subscriptions.
A large proportion of members only join for a month, download everything, and leave. This is fine, but it means we need to continue attracting new signups or we will be unable to pay our developers.
This is where the free extensions come in... they contribute the vast majority of our traffic. Some of this traffic gets converted to club signups.
In essence, without people coming here for the free extensions like Ninja Shadowbox, we would really suffer.
This is the reason for the club rule... no distribution. If other people started distributing Ninja Shadowbox, we would have less traffic, and everyone, not least our members, would suffer.
The rule is designed to protect both you, our members, and us, because we depend on it for our livelihoods.
Please do not distribute our extensions. I hope I have made clear how important that is.
Don't hesitate to ask here or email us at support@ninjaforge if you have any further questions about this.