The article is quite long, but these two give a brief idea of what is going on:

a) According to that article, what it seems they are about:

> Its manifesto describes the project in explicitly political terms. A/I is antifascist, anti-capitalist and antimilitarist. It promotes free software, encryption, pseudonymity and the independent circulation of knowledge and cultural work. It reserves its limited, volunteer-run resources for people and projects broadly compatible with those principles rather than businesses, political parties, organized religions or institutions already possessing the means to make themselves heard.

b) And the issue U.S. has with them:

> The United States alleges that A/I supplies specialized digital architecture, encrypted communications and hosting to antifascist groups and other radical movements, including organizations already designated by the United States.