Violation of their privacy right
"Brokers who trade such lists argue they are compiled from customers who agree to have their information used for marketing purposes."
No the customer agreed to let X use their phone number for marketing purposes. This is to define X as anyone and everyone for any purpose. How could they have agreed to let their phone number be used for a directory service that did not exist at the time? They could not.
The data watchdog is, as ever, a proxy permission giver. Giving permission to share information that the person themselves does not want shared.
You did not give them permission to add your name to a public directory, the data watchdog is doing that as a proxy for you. As though his choice overrules your choice. He should not be doing that.
If you want to giver permission then they could let you add your phone number to the site, however who would do that? If they wanted someone to have their telephone number, they would give it to them, not give it to a site for all the world to gain access.
The data watchdog should step in, or rather the PRIVACY watchdog should start getting injunctions.