The "they [=big corps] are the ones doing to most to improve the situation" bit -- it sounds almost as if you thought they're doing it all by themselves, outta the goodness o' their hearts.
No, they're not doing it out of the goodness of their heart. They're responding to the demands placed upon them by their government. In that respect, GP has done alot of good. HOWEVER, GP continues to force their issues on the larger, western companies when what they need to do is attempt to influence the legal system in the countries that are NOT as restrictive as we (US,ECC) are.
Big co's in the US now do NOT do the bare minimum that the law requires (by and large...there are exceptions). Most go well beyond what is required. Reason, unlimited liability. It has been shown (asbestos, arsenic, etc...) that what you did 50 years ago you can be forced to pay for now...even if the best information available at the time showed no ill effect from your actions.
So while it has been legislation that instigated it, and GP and other environmental groups have done well to influence it...it has become the bottom line that drives it. Sure its not "goodness of heart"...but you knew that already.
If it truly is the betterment of the global environment that GP is interested in...it will take information like was given above as affirmation that some improvements are being made...and focus their attention to places where they can really do some good as opposed to places where they can get the most press.