Should polo tell people they need to use a payment ID? yes. However, I just looked at my polo balance screen and saw this:
It would seem that they are doing what they can do warn users that they must use a payment ID. If the users are not worried enough to contact polo about their funds, why must polo spend hundreds of hours (paid to the employees, no less) tracking down people who didn't follow instructions and don't care enough to go after the funds?
Furthermore, fluffy is right. There is no reason the funds should go back to the developers. The funds should be returned to the people who sent them if there is a way to link them back. If not, polo should be able to keep them. I would like to thank polo for giving us a good platform to trade XMR upon, including using XMR as a base pair. Their platform is better in responsiveness than cryptsy, and the features are better than bittrex. If it weren't for them, it is possible that XMR wouldn't have as much exposure as it does now.