My guess is that you're 95% correct: they're likely warding off Verizon, who has become a major threat to all three companies. Offering FiOS TV, FiOS internet, phone, and wireless cell service is tough to compete with — even if you already offer three of the four.

So, these three companies are trying to prevent people from switching entirely to Verizon because they already have/are considering Verizon's wireless service. Or something to that effect.

(Note: I'm from the NY Metro area, so I've kept an eye on all these companies offerings the past few years.)