As I understand it, the test for monopoly is detrimental effect for consumers, not control of a market. Making your company more vertically integrated (by buying a back catalog in this case I guess) probably doesn't trip any alarms.

I'm surprised that they don't buy Penguin. Maybe Bertelsmann has no interest in selling.

If you think about it, publishers of print/music/movies are essentially built on monopoly given the single sourcing of a title. It's not as simple as cornering the market on wheat or oil.