Here’s what I found today:
The answer plain and simple is that they did so to make MORE MONEY. They were spending a specific amount of money per user acquisition and they needed to make sure the user lifetimes held up after they were acquired so they didn’t lose money. User acquisition costs were probably going up so they had to do what was in their control to manage that. Since they couldn’t influence costs, they tried to influence the average user life. That’s the real answer.
I have all the respect for Steve Case but he dodges the question on Quora and yet he gets up-voted for a low quality answer.