The fact is, network have affiliation agreements that guarantees exclusivity over the TV screen. Until digital revenue exceeds broadcast, don't expect that to change. In fact, recent cable retransmission agreements make it likely to never change.

As someone who works on post-production of network episodic dramas, I can tell you that Apple DOES NOT pay for product placement. They provide machines to set designers for free. The set designers love the look of Apple products. Hollywood is mostly Mac with the exception of render farms and certain workstations. I have had to "paint out" many Mac logos in many shows because the nets don't want to give free advertising. BTW, IIRC, Steve Jobs saw a Powerbook on a show with the Apple logo upside down and that is when he decided to the the logo so it would read correctly with the lid open.