The Veda-pUrva-bhAga is the forest in which the trees have bloomed, of which the flowers are the Upanishads.
Upanishads are like the final flowers whose fragrances are ready for any sincere seeker to take. The essence of these flowers has been sprinkled in purANa-s too, by the revered Veda Vyasa. There are no limits on who can smell the flower or the taste the essence of the flower. They should have that particular sense. That is it.
Giving back to the system (the vaidic system) may be interpreted as going out of the place one has smelled the flowers and running around in ecstasy letting people know where the flowers can be found. Some other people interpret the giving back to the system as choosing to remain in the forest, and taking care that the system that has lead to the flowers bloom is intact.
This constant gardening is mostly a thankless job and is error prone and leads to fear of staying within the realm of dharma, as the veda-pUrva-bhAga is full of kArmic injunctions which should be carefully dealt with. This fear manifests in anger towards anything that seems to obstruct the path. The constant watching of actions at the manas, vaak and karma so that there is no dharma-glaani occurs is a difficult task indeed. (Is this the reason why some of our ancestors were known to be very anger prone? Was it the anger which was fueled by the dharma-glaani, or was it the one fueled by kaama, as gIta says many times?)
There may be a selfish reason too: they get to smell more flowers than the ones who go out with limited flowers. Occasionally they need to take care of the serpents that are the the forest, but that is part of the job.
The people who run around about the fragrance often (often indeed!) meet people who do not have the taste for the flowers. These latter varieties of people are perhaps ones who lost the particular sense, and they may argue about the futility of the fragrance/taste itself (they could be like the characters in the "Country of Blind Men" by H.G. Wells).
This poses some questions:
How many are ready for the gardening of the forest? Who ready for this difficult task? How long can one keep smelling the flowers?
Alternately how long can one garden the forest? Doesn't one "give back" something to the forest? What if the forest is infinite? Shouldn't the fragrance be propagated by living it, instead of working on improving it?