They're not real losses. They could sell the movie. They could release it. They're deliberately forgoing those options and deleting it. You don't have a loss until you attempt to generate revenue and find that the revenue is less than your expenses. If you just throw your product away, that's not a loss.

Ultimately we write tax policy for the benefit of society, not for Warner Bros. In theory, we could write tax law so that it made financial sense for a factory to produce widgets and send them directly into an incinerator, because they could claim this as a "loss" and offset profits from other activities. But we shouldn't have a tax law that does that, because that's a huge waste of society's resources (materials for the widgets, fuel for the incinerator, people's time and labor). We could use those resources to do something more productive and tax laws should encourage companies to do productive things that improve the world, not spin their wheels for imaginary losses.

If you deliberately burn down your house and try to collect insurance on it, we call that insurance fraud. If you deliberately destroy your product and try to claim a tax loss on it, that should be fraud too.