It's not strictly rational but when you are giving extrinsic rewards, like a gift card, they are not perceived as an added reward but as a replacement. What did they replace? The intrinsic motivation/justification that prompted them to perform the action in the first place.
When the motivation switches from intrinsic (I'm doing this because I'm a good person) to extrinsic (I'm doing this for money) we use a different value of judgment which in this case appears to not have worked as well for you as you would have liked. Instead it may have been better to offer some form of recognition/acclaim to reinforce the intrinsic motivation and promote this behavior. For example helping people on stack overflow rewards you with feeling good about being a productive member of a community; the "reputation" score reinforces that same fact. Now imagine instead of having the reputation score mechanic you were paid 25 cents for every accepted answer instead? Would we have seen the same adoption or would people have not bothered "working" for a few dollars an hour?
When the motivation switches from intrinsic (I'm doing this because I'm a good person) to extrinsic (I'm doing this for money) we use a different value of judgment which in this case appears to not have worked as well for you as you would have liked. Instead it may have been better to offer some form of recognition/acclaim to reinforce the intrinsic motivation and promote this behavior. For example helping people on stack overflow rewards you with feeling good about being a productive member of a community; the "reputation" score reinforces that same fact. Now imagine instead of having the reputation score mechanic you were paid 25 cents for every accepted answer instead? Would we have seen the same adoption or would people have not bothered "working" for a few dollars an hour?