Why did the us military throw all equipment over board when returning from Vietnam?

Answer 1

Not everything was thrown over-board. Some of it was buried by engineer tanks or dozers. Generally speaking:

1. Vietnam was fought by US WWII veterans; and that's what they did in WWII, shove the stuff over-board, especially if they needed the room for incoming aircraft or other uses.

2. With the exception of what was termed "big ticket items" such as tanks, field pieces, etc. everything was considered "expendable." Civilians seem to get extremely upset (and not because of tax dollars either) when the army shoots up a tank at the tank range, or the navy sinks an old warship for torpedo practice (formerly gunnery practice). But to soldiers & sailors, IT'S GOOD TRAINING!

Bottom line: unless, as stated before, it's a big ticket piece of equipment, everything is expendable in the military; war is the profession, and in wars (20th century wars anyway) tons and tons of what civilians would call good surplus store/gun show "Stuff" is shot away, buried, sunk, or blown up. War is destructive, not constructive, and training has to be as realistic as possible. The military used to train in the 20th century with this slogan, "Sweat more now, bleed less later ( bleed less in combat)."