It has nothing to do with the iPhone at all and your timeing is off for when they got the iPhone and when they started the ETF. It has everything to do with the greed of these carriers.
It had everything to do with the iPhone, at least for Sprint. Sprint released the iPhone 4s on October 14, 2011. Per Sprint's terms of service, phones purchased on or after September 9, 2011 are subject to the $350 early termination fee, while phones purchased before that date . Sprint also once had a 30 day return policy that they shortened to 14 days in mid-September as well, and they eliminated their Premier program and "no cost" trial period for new customers at the same time. This effectively prevented any new customers from getting to use the iPhone for free for 30 days, which would have been allowed under the old policies, and ensured that even old Premier Gold subscribers (like myself at the time) would be locked into the iPhone for a full 20 months if they upgraded to it. It also eliminated the possibility of someone grabbing a "hold-over" phone for 30 days right before the change and then returning it for the iPhone in October.
While they never flat out said all these changes were in regard to the iPhone's release, the consensus on forums and the press was that the iPhone was the reason. Sprint devoted a ton of money to the iPhone, and they needed to help ensure some return on their investment. Is that greed? Perhaps, but I call it smart business.