The other day someone was asking why Facebook wants people to adopt to React, which is generally just a question about why all of these software vendors want to eat the world and become the source of everything that commands user attention.
This is why. Apple can use its influence to harm its competitors by frustrating the mechanisms used to promote their monetary interests, making a (legitimate) claim that doing so is beneficial to the user.
Microsoft articulated the goal well in its 1998 memos, the infamous "Embrace, Extend, Extinguish". They want platform control because once you control user attention (and developer attention by extension), you can wedge your way in and do things that benefit your company over others.
Facebook controls the front-end library used by a large portion of all sites, not just facebook.com. If Apple's machine learning mechanism notices some common patterns between Facebook's code for React and Facebook's code for user tracking, either organically or because Facebook does a couple of nudges to make sure that happens, now Safari is broken, evidenced by its inability to correctly render a large portion of the web. If Facebook only had control over code running on facebook.com, then Facebook would be the apparent source of any user-facing breakage.
Note that users are not the customer being catered to. They are, rather, the resource being exploited to provide the energy necessary to undergird the MegaCorp's power expansion, in service to the other MegaCorps doing large-scale advertisement, endorsement, and censorship/speech control deals with them.