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.)
Makes sense to start in an area where some of the big players have territory that overlaps.
To be clear though - companies like Clear and Sprint aren't just selling 4g wireless devices, they are selling hotspots filling the roll of routers that supplant a home internet connection all together.
When Verizon gets in the game - maybe FIOS for heavy need households, 4g hotspot for a light duty setup?
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.)