iTunes music accounts for 70% of legal music downloads.

Size does not matter when defining a monopoly. Monopolies restrict trade. There are many services that are actually equal in price or less than iTunes with similar catalogs. Definitely not a monopoly.

Also, just because one part of the business is a monopoly doesn’t mean that other parts of their business are monopolies.

No, the mere possibility of restricting trade makes a "monopoly". Actual restriction of trade is "abuse of monopoly".