Under what law are apple's actions prosecutable?

The problem with this whole mess is that monopoly, monopolization, and monopolistic practices were not defined by legislation. Legislation in fact provides no definitions whatsoever. The entirety of Section 2 of the Sherman Antitrust Act is comprised of a single run on sentence.

It's all well and good to be all "it's Mabo, it's justice, it's the vibe" over this, but congress hasn't provided legislation required to regulate competitive markets.