The question of whether theories of obligation should be derived from statements of "is" or "ought" emerges out of a long-standing, and as-yet-unsettled dispute over epistemology.  Empiricist approaches stress the conception of obligation derived from "is"- the visible and verifiable material world.  Non-empiricist (?) approaches seek moral obligation derived from reasoned deduction of moral premises, emerging not from a verifiable material world, but from conceptions of 'best' systems of moral obligation.  The debate can be understood well in context of the conflict opinions of Thomas Hobbes and Immanuel Kant. 

Hobbes

In Thomas Hobbes deduction of society,  he sees society in terms of what "is" and what "ought " to be. For Hobbes the "is" is the current situation/reality and the "ought" is the proper moral obligation everyone has to the "is", or current situation. In order to hold society in balance the "ought" has to fix the "is". The "is" is existence, the "is" is real, the "ought" is how the "is" should be in the perfect form. In order to discover what the "ought" should be, Hobbes believes it is necessary to determine what holds society together. (This is essentially the first sociological question. "What holds society together?") Once it is understood what is holding society ("is") together, then the "ought" (morals) can be determined and intervene.

Because Hobbes sees the state of nature to be intrinsically self-motivated, there has to be the "ought" that will keep the self-motivated society ("is") from falling apart. Hobbes believes markets to be the best institution for a self-motivated society, because a market is an exchange of ownership; governed by self-interest, one can decide to sell or not based on their own interests of what is best for them. Therefore, their has to be a state sovereign to hold the market together, that will be the "ought" to the "is" of market society. This state sovereign does not have the power to intervene in the market's dealings, but rather only to prevent corruption and injustices in the market, in order to hold the market together.

Kant