It's a shame that that is their policy. I respect the right of any company or organization to do/not do business with whomever they wish. But their reasoning of guns promote suicide is just silly.
I would have more respect for them if they said we don't do business with the firearm industry because firearm ownership can be taken as a political issue and we do not want to even come close being regarded as politically motivated. At least then, they may have some basis for their claim. But then they would have to say we don't do business with abortion clinics, etc. But I digress.
After looking at the link the OP present,
www.charitynavigator.org, it looks like there are better organizations to donate to other than the WWP, anyway. It looks like their workers take a big chunk of the donations that come in now add that with their silly explanation of firearms, I won't donate to them.
Edit: I bet if they were to promote inside the firearms industry, they would make a killing in donations. Think about it. A Colt 6920 Magpul/WWP edition. Half of the profits get donated to WWP. So many people would buy that gun its silly to think it would stay in stock.