I think the default channel a Sim turns on the TV to depends on their traits and current emotion... for example, if a Sim is flirty, they might gravitate towards the romance channel... if a Sim is athletic, they might gravitate towards the sports channel... but, if a flirty Sim is energized, they might turn on the sports channel, while an athletic Sim who's feeling romantic might turn on the romance channel.
That's how I've personally observed it... I have no idea if this is actually how it works, but that's almost always the case with my Sims.
