It's not setting the fire which causes taxpayers to lose money, it's writing off the loss against taxes you would have otherwise paid on profits made from other movies.
Not an accountant, but tax write-offs aren't some sort of no tax glitch that you make them seem. You can only deduct losses that you've actually suffered, not imaginary losses. If you spent $10M into making a movie, you can't value the movie at $80M and then use that against all your other revenues to pay less taxes. If this were true every company would be making fake movies to destroy and pay less taxes, not destroy actual movies that they spent millions on.
Taxing people on income they didn't make but the government imagined they could have made in some situation that didn't happen sounds like a slippery slope to be standing on.
