"Direct" is the term the judges have adopted to refer to the infringement that is explicitly defined by statute, due to the necessity of distinguishing it from the common law derived categories of contributory and vicarious infringement. The statute does not need to use the term "direct infringement" for that term to have real legal meaning.

I've already quoted the Supreme Court on this elsewhere in the thread. The MGM v. Grokster case (2005) included:

> When a widely shared product is used to commit infringement, it may be impossible to enforce rights in the protected work effectively against all direct infringers, so that the only practical alternative is to go against the device’s distributor for secondary liability on a theory of contributory or vicarious infringement. One infringes contributorily by intentionally inducing or encouraging direct infringement, and infringes vicariously by profiting from direct infringement while declining to exercise the right to stop or limit it. Although “[t]he Copyright Act does not expressly render anyone liable for [another’s] infringement,” Sony, 464 U.S., at 434, these secondary liability doctrines emerged from common law principles and are well established in the law, e.g., id., at 486. Pp. 10—13.

( https://www.law.cornell.edu/supct/html/04-480.ZS.html )

The court is citing and quoting an earlier decision, Sony v. Universal City Studios (1984), the landmark case establishing that it was legal to use a VCR for timeshifting. That decision made numerous references to a distinction between direct and contributory infringement, such as:

> Justice Holmes stated that the producer had "contributed" to the infringement of the copyright, and the label "contributory infringement" has been applied in a number of lower court copyright cases involving an ongoing relationship between the direct infringer and the contributory infringer at the time the infringing conduct occurred. In such cases, as in other situations in which the imposition of vicarious liability is manifestly just, the "contributory" infringer was in a position to control the use of copyrighted works by others and had authorized the use without permission from the copyright owner.

( https://www.law.cornell.edu/supremecourt/text/464/417 )

And that's referring to a 1911 decision by Oliver Wendell Holmes Jr., Kalem Company v. Harper Brothers ( https://www.law.cornell.edu/supremecourt/text/222/55 ).

So "direct infringement" is a pretty well-known concept despite not being named as such in statute.