You frame the situation in an odd way. As going carbon neutral is somewhat altruistic, it would take a fairly superficial or image conscience organisation to seek direct benefit. Let's frame it a different way. If you were a board member of a chemical plant that dumped heavy metals into the local creek you could also be asked "what's in it for us to not do this? Why should we stop polluting? Just to feel good?"
In addition to the legal difference already mentioned, there is another key difference. It is possible to completely stop dumping heavy metals in the local creek. It is not possible to completely stop carbon emissions. We can reduce them, certainly, but not eliminate them (offsetting is at best a stall tactic).
With the chemical plant there is a finite cost associated with stopping dumping and the company can simply decide to spend that money. With carbon emissions there is an unlimited cost associated with reducing emissions with ever diminishing returns. It's not a matter of deciding whether or not to spend the money, it's a matter of deciding how much to spend. Should we reduce emissions 10%? 50%? 90%? It becomes necessary to assign a numerical value to the reduction in order to determine how much it is cost effective for us to reduce by. How do you suggest we do that?