I might write this up in broader terms later with pointers to some social psychology studies, but I just got back from a long series of family gatherings and don't have the state of mind to focus too much.
The standard story in psychology about "deindividuation" -- losing yourself in the crowd-feeling, no longer very aware of yourself as a cut-off individual -- is that it leads to all kinds of harmful activities. When the mob gets together, accountability is diffused over the entire group, so no one feels so responsible themselves, they're in a state of physiological arousal and thus pumped up to do something big, plus they're so group-minded that they are less able to monitor themselves internally and regulate their individual behavior within acceptable norms. Hence all of the spur-of-the-moment violence that mobs are capable of.
However, losing your self-consciousness lets you step outside of your normal boundaries not only in the anti-social but also the pro-social direction. Normally we might be penny-pinchers when it comes to charity, when suddenly we become absorbed in the crowd at church services and give much more liberally to the collection plate than we would in our normal state of mind. Especially if someone else has already gotten the ball rolling, and we're just going with the flow of the group.
It's no surprise, then, that fund-raisers try to create a carnivalesque kind of atmosphere: dim or dark lighting, seating people near each other and facing each other in groups, working them into arousal with catchy music and humor, and encouraging them to dress similarly rather than compete against each other in dress styles. Getting them drunk helps too.
The other place I found it easy to donate -- as in, no one needed to ask or pressure me (as they do at the supermarket check-out, or at Wikipedia) -- was at a diner after we'd eaten and went up to the cashier to pay the bill. Get the customers worked up into that self-unaware state of mind as they shoot the shit and laugh it up during their meal, and place a collection jar near the cashier.
Are the donors being exploited by fund-raisers, or at least "nudged" paternalistically by the collectors? Here and there maybe, but on the whole they must realize that in those situations they tend to give more than they normally would -- they may even express surprise to themselves after leaving the crowd and coming back down to normal life. Purposefully or not, they must be seeking out opportunities to donate more, or donate at all, where they won't have to self-consciously reflect on whether or not to give, what the consequences would be, and so on.
They want to shut up their inner Scrooge for awhile, and only losing themselves in the crowd can do that. If this loss of self-monitoring and self-regulation lasted forever, they'd "splurge" on charity all day every day and ruin themselves, but we're only talking occasionally here.
The crowdophobes only focus on the harm that impulsive mobs can carry out, neglecting the far more frequent and substantial altruism that comes from such an atmosphere. In the real world, outside of lab experiments, for every mob that torches a car, there are dozens that gather in church and provide mutual aid of one form or another. We just have to get used to taking the bad with the good in encouraging more activities where people lose themselves in the crowd. The alternative is to a high level of self-consciousness, which will keep us from getting whipped up into a violent mob, but will also keep us from working ourselves up into a hand-lending mob too.