I would suppose for any extension to work with Facebook the site it would need to be developed with knowledge of a Facebook page's DOM, as such the developer would need to go look at the page to be used and write code to do what the extension needs to do.

Thus I guess one legal argument would be that the ability of the extension to work proves the developer's use of Facebook the service even though they have been banned. So maybe there is something from that they can build up an argument, although it starts to sound far-fetched enough I might expect a judge to not buy it.