That is, saying that customers /could/ switch is not necessarily relevant. There is evidence that they believe paying for the "default" billing is worth a ton of money. Not just to Apple, but to Mozilla, too.
That they could lose access to revenue streams by losing default billing is very relevant when that is also a large portion of how they are able to make that payment.
Could see it more as collusion with competition, I suppose? It isn't competing on the merits of their search, but more on their ability to afford default status. Which keeps Apple from entering search, and ostensibly keeps it more expensive for Microsoft to keep Bing.
Consider, if Apple could get more from MS to make Bing default, why wouldn't they?