If Apple charged everyone 30% and doesn't itself compete in music streaming, that's fine. Everyone is playing on the same field and by the same rules. The problem is Apple is using its market strength in app stores to advantage itself in the market of music streaming because it isn't subject to the same 30% fee and restrictions on advertising.
Just think about the math: Let's say a Content Owners charges $2.50 to anyone who wants to offer it for download, and both Apple and Competitor offer it to download. Apple sets the price at $3. If Competitor sets the price at $3, it has to pay Apple $1, and thus only gets $2. Thus, at the $3 price point, Competitor loses $0.50 per download AND Apple makes $1 for each of Competitor's downloads, and Apple makes $0.50 for each of Apple's downloads. Competitor can't price it higher than Apple, because they would lose customers.