It's not an assumption, it's commonly done in some parts of the world that don't have proper waste management. What happens after you put trash in a bin and there's nobody to collect it? You have to get rid of it somehow. Out of many options, such as throwing it in the sea, burring it in the ground or simply burning it, the latter is usually done because it's easier and quicker.

Heating isn't really an issue in the part I lived (warm all year round, even in the night) and the smell of burned plastic is quite distinguishable, even at higher altitudes.