calisthenic

Regarding starvation mode (I know you didn't want to talk about it, but I personally wanted to address it), it depends on an individual's energy expenditure, and how much they eat. Your energy expenditure is broken up basically by: physical activity, Thermic_effect_of_food) thermic effect on food, and your basal metabolism. Your basal metabolism is basically all the energy your body requires normally, without you doing anything. For example, if you decided to sleep all day, you would still need to eat a certain amount of food for your body to use to do its daily activities (keep lungs inhaling, bone marrow making new red blood cells, etc). The "thermic effect on food" is basically the energy required for you to digest and process all the food you eat in a day.

The cause of "starvation" is an imbalance of energy intake, and energy expenditure. This means, if you keep eating less than your body requires (from its BMR, theramic effect on food, and physical activities), it's going to start doing Catabolysis to stay alive. This means, it'll use its stored fat, energy, bones, muscle, etc. to give itself its necessary energy intake, in order to keep you alive.

Everyone has a different BMR, because your BMR is factored by so many things: age, height, growth, gender, etc. A baby may require only 500 kcalories for its basal metabolism, whereas a man might require 1500. In order to figure out your BMR, google "BMR Calculator" to get rough estimates (I can't tell you how accurate they are, currently). People eat a certain amount of food a day, adding to your energy expenditure. Also, everyone does different activities throughout the day in which adds onto the BMR, and the thermic effect, dictating your energy expenditure for the day entirely.

On a long-term diet where your body continuously does Catabolysis, eventually you may experience symptoms of "starvation", such as lowered body tempature, impared vision, organ failures, depression, food-related dreams, etc. Therefore, it isn't entirely "BS" that one could go into "starvation" on a diet of 1200 or less. It, overall, just depends on their individual energy expenditure, and their daily diet. If their BMR is 1500, and they burn 400 kcalories a day from exercise/moving, (1900 kcalories of energy expenditure) and they eat 1200 (it takes energy to digest food, therefore about 120 additional kcalories, bringing total for day 2020), then perhaps they could experience symptoms of starvation, just, not in a good while -- perhaps years, or maybe months. (I'm not sure how long, I am speculating on that part), or, if their energy expenditure is 2020, and they eat 500 kcalories a day, they will probably experience starvation a LOT sooner.

Anyways, in regards to your actual topic, I think you have the word "burning" confused. Any kcalories you "burn" in a day will be added to your BMR and thermic effect, rather than subtracted. For example, if my BMR is 1500, and I did a workout that required 500 kcaloires of energy, and I ate food which required 200kcalories of energy from my body, then the overall energy expenditure I did for the day is 2200 kcalories, meaning if I did that everyday, and I only ate 1500 kcalories a day, I'd eventually lose weight (but this isn't the weight loss board, so I wont get into all that).

Conversely, if I kept doing a 2200 kcalorie a day energy expenditure, and I kept eating 1500 kcalories a day, how long would it take till I reach symptoms of starvation? I couldn't tell you, but, again, starvation is from an imbalance of energy in and energy out. If you did a 2000 day energy expenditure, and you only ate 1000 kcalories a day, you'd reach "starvation" symptoms probably a lot faster.

TLDR; Your question was: How is it that eating less than 1200 calories a day equals starving, but burning off several hundred calories (or burning more than you consume) is not?

My answer: It equals starving because (saying your energy expenditure is more than 1200 kcalories a day) you're eating less than the energy your body is burning. When you start experiencing signs of starvation is dependent on your body, time, amount of energy expended in a day, and how much you're eating.

Burning more than you consume follows the same concept. A long-term imbalance of energy in and energy out could eventually show symptoms of starvation -- depending on time, the body, the amount of energy expended in a day, and the diet.