The disposal of C-ration cans was not something that was universal by any means. Those familiar with modern day military operations in the field would presume that most soldiers have either carried the trash with them or got rid of it through some other means in which they wouldn't leave a trail after themselves. It's never wise to project modern day thinking (which works in a specific present context) into the past (with its own specific historical context), even if it's something so recent as the Vietnam War.

Let's start with the most common way of getting rid of C-rations cans: You buried them. The notion of burying food containers, empty or not, is a theme very apparent throughout the Vietnam War on both sides. It was an efficient way to get rid of trash for Americans, a way to avoid food falling into the wrong hands for civilians, and to supply troops on the move for insurgents. For a US soldier in the field, having to carry around empty cans was not an attractive prospect due to the simple fact that it would be noisy. Imagine a platoon where each individual carried several empty cans. It wouldn't be a subtle noise heard throughout the jungle. Instead, it was easier to, as Marine Recon Steven A. Johnson writes, "to bend them as flat as possible and bury them." Here's a description by another Marine, Federick Fenwick, of how they would get rid of trash:

On the third day in this position the word was passed down through the chain of command that we would be moving. A working detail from the various platoons dug a big hole for the entire company to bury their trash. I was part of the working detail. Each platoon gathered up all the company's trash that included empty C-Ration boxes, cartons, and open and unopened C-Ration cans. Marines that just did not like the contents like beef and potatoes, for example, threw the unopened cans away. If they threw away unopened cans they were supposed to punch holes in them with their Ka-Bars or bayonets. This would ensure that the food would spoil before the VC could get to it. There were always some cans found that had not been punctured because the Marine discarding the can was either too lazy or just did not care. A fire was set up to burn the trash. After the blazes subsided the working detail covered the hole by throwing in dirt to fill it up. Then they returned to their assigned platoons and we prepared to move out.

The above description gives us an idea of a much larger disposal of trash than would have been common for a platoon, in this case a company. It also makes an interesting observation that brings us to my next point: There were soldiers who were too lazy or just didn't care where their trash ended up. Of course, we can not simply attribute this to soldiers being lazy either. In an orderly fashion, trash was meant to be disposed of by crushing and then burying it in a pit which would then be burned. This was the best case scenario and the scenario that most most sense. Yet we can not forget for a second that this an act that involves human beings. That meant that soldiers forgot to bury them times, were too tired to even care, or simply found themselves in a hurry.

No matter if you did it the right way or the wrong way, the chances were that the cans would be reused. From civilians using the remnants of cans and other garbage to reinforce their homes to insurgents using empty cans to create make-shift mines or other booby traps, the trash that the US soldiers left behind them came to have a life beyond simply containing food.