The free market isn't a simple two-way exchange of favors, it's a system governed by supply, demand and the value of goods and services... not reciprocal altruism.
If an individual works for free, they are not creating a reciprocal obligation on the part of a corporation. In a functioning market, companies pay for labor because they benefit from it. If someone chooses to work for free, that's a personal decision... not a transaction that obligates others to reciprocate.
