Clearly you don't understand the concept of social signalling.
If they keep the money (even if they donate to a good cause), it allows adversarial PACs and organizations to make claims like "NAACP is a hypocritical organization for taking this money from Facebook". These will be more costly in PR to counter than the $1mm donation, and ultimately harm their cause.
In this situation, the only appropriate action is to give the money back, or not accept it in the first place.
I think this is an excessively cynical interpretation: sometimes people and organizations just return money because they think it's ill-begotten, not because they've calculated that keeping the money will either harm their reputation or cost them more than it's actually worth.
In a sense, making a public Twitter post, blog post, and hash-tag movement is leveraging that donation, even more than it's monetary worth, to advance the NAACP's own incentives.
So in that sense, they did indeed take advantage of the donation, not in the first-order monetary sense, but with the resulting publicity and press that probably surpasses the donation's intrinsic value.
Pretty good move on there part to be honest. I wonder what would've been the public outcome of them re-donating that amount instead to one of them "Soros-funded" anti-Facebook organizations as a "screw you".
If you disagree with someone KEEP their money!
IF Satan himself gives you $1M just keep it and use it for something good.
If you give it back he's just going to use it for something evil. At least you're using it for something good.
I mean there's the argument that now satan can now claim that he supports your non-profit but I think you can do more good than the bad...