It's not formal law, but informal warnings straight from congress and who knows what else privately. Ex: The current investigation into big tech poo-pooing FB's acquisitions of WhatsApp & Instagram, who say that under current law that there isn't anything illegal going on, but maybe we, congress, should make that kind of stuff illegal too.

Everyone has learned from microsoft's antitrust trial you want some semblance of a competitor and not look too self serving.