Carbon Offsets: Who Should Be Picking Up This Tab?

Last month I purchased a round trip airplane ticket online for a trip to Denver from Detroit. At the end of the transaction, Delta airlines invited me to contribute $11.74 to the Nature Conservancy to "offset your share of carbon emissions for this trip." The distance traveled will be approximately 2,300 miles.

Maybe I am missing something here. But who is the carbon culprit in this case? Shouldn't Delta, who is producing the carbon emissions and making a profit in doing so, be making the carbon offset donation?