All addons are required to be free to access, download, and use. Blizzard does not allow people to put those addons behind any kind of paywall to download or use. Paying real money to join a guild so you can use their addon scripts is very much against the rules. They are selling something that uses Blizzards API data and lua sandbox.
So if they are doing that, it would certainly be against the rules. There was a whole thread in GD about it yesterday but I think the mods might have nuked it.
Applications May Not Charge Premiums To Use The APIs. “Premium” versions of Applications offering additional for-pay features are not permitted, nor can players be charged money to download an Application, charged for services related to the Application, or otherwise be required to offer some form of monetary compensation to download or access an Application when those features use the Blizzard Developer APIs. Likewise, Applications may not include interstitials soliciting donations before features or functionality becomes available to the player. Also, players may not be required to watch a video advertisement before accessing a feature or function of the application.
Also reflected on the UI and addon forum for easy reading.
World of Warcraft User Interface Add-On Development Policy
With the continuing popularity of World of Warcraft user interface add-ons (referred to hereafter as “add-ons”) created by the community of players, Blizzard Entertainment has formalized design and distribution guidelines for add-ons. These guidelines have been put in place to ensure the integrity of World of Warcraft and to help promote an enjoyable gaming environment for all of our players - failure to abide by them may result in measures up to and including taking formal legal action.
- Add-ons must be free of charge.
All add-ons must be distributed free of charge. Developers may not create “premium” versions of add-ons with additional for-pay features, charge money to download an add-on, charge for services related to the add-on, or otherwise require some form of monetary compensation to download or access an add-on.