Do bananas ripen better in rice sacks?
Yeah, I would agree with the people in the comments, and make the assumption that it had to do with concentrating the ethylene gas from the bananas. 

After a bit of googling, it looks like the rice bag trick might be popular because it apparently results in a nearly ideal, stable environment for ripening.

I imagine that the rice would keep the moisture level at a lower point, the bag would contain the gas, and the mass of the rice would decrease the temperature variants (assuming that is conducive to ripening)

I found an informative article which mentioned rice bags here:
http://monsterguide.net/how-to-ripen-bananas-quickly