Carbon comes from a bunch of different sources and I've never seen it listed as a fertilizer component, even though any fertilizer that contains sugars obviously has C in it.

The soil, air, and even to a certain extent the water all carry Carbon in some form. Molasses is simply a nice safe way to guarantee that your plants have a good source that they can uptake through the roots. Go ahead and add some molasses, a couple tablespoons per gallon is PLENTY, and you don't need to add it at every watering.

My molasses schedule:
1 glob per economy-sized laundry detergent bottle approximately whenever I remember to/ feel like adding it... glob size is dependent upon ambient temperature... Oh, and I add sweet or topmax at a slightly lower dose than the manufacturers recommend along with teh molasses.