It's still not a First Amendment issue, because
government isn't keeping anyone "outside the gate" of the Mac App Store.
AMENDMENT I
Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the government for a redress of grievances.
Apple's behavior might be in violation of antitrust laws, but it's not unconstitutional.
