When you heat it, the rate of reaction speeds up so the THC gets activated faster because the particles are moving about faster, therefore there is a higher chance they will collide and with the right force and shape to activate the THC.
et al.
The truth is that THC is stored inside the cellulose (green stuff) of the plant as well as the trichomes. Compare weed to celery; when you eat celery, your body cannot digest it because it is mostly cellulose, which humans cannot digest. The THC is stored inside this cellulose, and therefore cannot escape the cellulose walls by digestion, and go to waste. Trichomes have a waxy covering on them, which is something else that us humans can't digest. THC however is fat and alcohol soluble (anything non-polar), which is why green dragon and edibles are made using oil (fat) or alcohol. THC IS NOT HEAT ACTIVATED. Heating the butter while the weed sits in it simply speeds up the rate that the butter absorbs the THC from the weed. If you try to dissolve one teaspoon of salt in a cup of water at room temp it will take a lot longer for the salt to dissolve than if the water was near boiling point. Think of the water as the oil and the salt as the THC. Anytime that people claim to get high by eating pure weed, it is all placebo effect.