There are over six million people in US prisons, only a small minority of which are in there for violent crimes. You're 8.5x more likely to go to prison for a non-violent drug offense such as possessing a trivial amount of drugs or even living in the same home as a drug dealer and being charged as an accessory. Your are 6x more likely to be in prison for a public order or "victimless" crime such as prostitution than a violent crime. You are 2.5x more likely to be in prison for a "weapons violation" in which nobody was hurt than you are for a violent crime.
So why is it OK to be happy about the prospect of people in prison being killed in a fire?
One solution to many of the technical, administrative and financial problems of running prisons would be to imprison fewer people. Canada imprisons less than 1/6 the fraction of its population than the US, and it's not a crime-ridden hell hole; Germany 1/9th and Denmark 1/10th the US incarceration rate. We could half our prison spending and spend the money on education (or give people a tax break if you prefer), and still have one of the highest incarceration rates in the world.
Why do we have so many people in prisons? Well, putting people behind bars is good for a prosecutor's career, especially if he has political ambitions. Also in states with privatized prisons the taxpayers are financially penalized for having occupancy rates less than 95 or even 100%. Think about that. Your prisons are overcrowded, so you hire a politically contractor and build a virtual guarantee into the contract that prisons will remain overcrowded.
Anyhow, a coarse net wouldn't rain fire down on prisoners. Stretch a piece nylon (very flammable) rope and try to ignite it by throwing burning stuff onto it. Even if it does catch it will only smolder. So net would be cheap and practical, which is precisely why it would never be used in the US: not enough profits to prison operators.