"Hidden" in the linked article it refers to gov't pressure to remove content.
"Tech companies have long resisted outside intervention in how their sites should be policed, but have come under increasing pressure from Western governments to do more to remove extremist content following a wave of militant attacks."
Comments here specifically refer to the U.S. First Amendment, so if the "Western governments" refers to countries exclusive of the U.S., then those comments would still apply, but the gist is that the tech companies are doing this in response to some sort of gov't instigation.
*If* the U.S. were involved, how does that fit with "Congress shall make no law"? Not too hard actually, how many things are being done now that are not directly linked to a new federal law? An executive order, or even a letter from some agency can do much, without any direct link to a new law. Hell, all it would take in most instances is a group of politicians blaming YouTube, Twitter and Facebook for various terrorist attacks to get them to do it, no forced of law required.