"I don't get why it matters if girls buy nothing. How does it cut into boys sales if girls who buy nothing are watching?"
You're not thinking like an executive. If Girls aren't monetizing while watching my Boy shows, that's just stealing time that the Girls could be monetizing watching Princess shows.
But, you say, you could learn to monetize Girls who watch Boy shows!
(At the point, the exec has already wandered off to yacht polo, or somesuch, but if they exec was still around,) "Disaster! I'd need to find a toy company that sells Boy toys, Girl Toys, and Girls-Who-Watch-Boys-Shows toys. My head hurts already.
And then my proper Girls shows are going to take a ratings hit because I fragmented my audience in these strange, unnatural ways. Get security up here! And bring me another caviar pop-tart!