This is a court case, so the Justice Department will have to use evidence to prove their case according to the existing laws. That's different than what is going on in the EU, where it's mostly legislative action that doesn't require any proof of anticompetitive behavior.
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For comparison: https://www.opensecrets.org/orgs/apple-inc/summary?id=D000021754
https://www.opensecrets.org/orgs/microsoft-corp/summary?id=D000000115