The commission seems primarily concerned with the potential impact of a new Apple streaming service on existing, ad-supported services like Spotify. They are worried that Apple could use its size to get labels to leave Spotify, according to the Financial Times:

The commission, which also has contacted Apple's music-streaming rivals, is said to be concerned that the company will use its size, relationships and influence to persuade labels to abandon free, ad-supported services such as Spotify, which depend on licenses with music companies for their catalogues.

Jimmy Iovine, a co-founder of the Apple-owned Beats Music service, has been talking to artists about bringing exclusive material to Apple's music services. Whether these deals would mean that they would leave other services behind is unknown, but nothing has emerged to indicate that they would do so. And while Apple's rumored service is said to lack a free tier, the free version of Apple's iTunes Radio is said to be sticking around.

Source: Financial Times