What a shame, and I would also like to appologize on behalf of rescuers that are NOT animal rights wingnuts for those that kinda give the word 'rescue' a bad name..... I am a small in-home rescue and the local "crazy bird lady", who has a great support system for wildlife since I'm not licensed to raise them when they're brought to me. I'm also on the Board of Directors of our local humane society, and foster HS cats in my home. Not everyone out there that has 'rescue' or 'rehab' or 'shelter' is crazy, though it does seem to happen more than I'd like to admit. I think that some people that deal with animals as employment or "hobby" tend to forget how to relate to people.
I gotta ask though, did you fill out any surrender forms? Any relinquishing of ownership papers? Typically larger organizations cover their behinds with paperwork like this, in case someone surrenders an animal and changes their mind and wants the animal back after it has been adopted by someone else, etc......
If there was any paperwork acknowleging that you were giving these ducks to these people, while what they did may have been morally wrong, it probably isn't legally wrong. Which is unfortunate, if feed toxins was the cause of their issue.