The lawyers behind the case maintain that customers have a direct relationship with Apple (and thus an antitrust case) since they send money to that company, not individual app developers. The lawyers further argue that Apple has extensive control over prices, until the point prices can end in 99 cents.
Do not expect to learn the outcome for a while. A decision on the matter is expected late in the spring. And if the Supreme Court allows the matter to go forward, it may be longer than before it is a judgment. However, if the lawsuit succeeds, it can have significant consequences for how Apple operates the App Store in the United States. Apple may need to reduce its share of app revenue, or provide customers with easier ways to find apps outside of the App Store.