Pretend that you are an internet provider looking to extract revenue from all possible sources. Only your direct customers give you money, so any time you can reduce carrying traffic to and from non-customers, you will.

Zero-rating your own (or subsidiary, or partners') traffic is a way of increasing the perceived costs to your end-users for traffic which is not as profitable to you. That reduces demand for the less-profitable traffic, thus allowing your profit margin to increase. Meanwhile, you get to spin the message to customers as "this stuff is free!" rather than "we built the cost of this stuff in to your monthly bill, and anything else gets counted against your quota".