Frontier AI labs legally can't coordinate development pauses. Why? Antitrust law treats this as potential market collusion. When competing companies agree to collectively stop or slow innovation, it triggers Sherman Act concerns regardless of stated safety motives. The legal framework doesn't distinguish between price-fixing cartels and coordinated R&D freezes - both restrict competition. Even informal agreements or industry-wide "voluntary" pauses create liability exposure. This matters because recent AI safety proposals suggesting synchronized capability thresholds or joint development halts face fundamental legal barriers in the US. Any binding coordination mechanism would need explicit antitrust exemption from Congress, which doesn't exist. The irony: safety-motivated cooperation gets treated identically to anti-competitive behavior under current law.