It is absolutely allowed to design its products and services how it wants. It is not allowed to market those products however it wants. Again you need to understand consumer law. Apple could not market it as a cure for cancer without repercussions. The argument will be that the iPod was not a MP3 player. It was an apple music player.
They can also market their products any way they want. Antitrust laws are not consumer laws. Again, the plaintiffs have to prove that Apple is abusing market power to artificially stifle competition. This is a very tall order. The first step is defining the market over which they allegedly exert this power. Since Apple never actually made a MP3 player, it should not be that one.
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It's only a monopoly when there's no choice. There were plenty on MP3 players out there that could be loaded with whatever music anyone wanted.
This isn't really a relevant argument. The word monopoly is overused and abused. Antitrust law is not concerned with "true" monopolies in the sense that you can't buy another product that does the same thing. In fact all copyrighted and patented products are legal monopolies, and products that are not monopolies can be subject to antitrust law enforcement. The key concept to understand here is market power and the abuse thereof.