Sell em for cheap. Hell even give em away to a charity or church. But why, why o' why would you smash em and throw em away. :@
Throwing them away because of the accounting implications.
Not in this case, Bob. People walk off film sets with a lot of stuff - much of it just given away. This would definitely have been the case with smashed guitars. The intact stuff they do sell off super-cheap, and they ship back anything that (a) may be needed for promotions or sequels (1 instance of Ace Ventura's outfit) (b) may acquire value if the film is a real hit (Luke Skywalker's lightsaber).
But a lot of wardrobe is simply given away, as are minor props.
Maybe it's a factor that everyone is on short-term contract, so the "consideration" isn't a factor? Or because foreign companies have different tax requirements.
The reality is simply that out of an entire film-crew, no-one knew enough about guitars to want those parts, or even to pre-book the amplifiers before the general sale (as crew are able to do, if they sweet talk the department head concerned).