I agree with you, in principle. That's how it should work. Unfortunately, most people don't do that for whatever reason. This sort of campaign is basically free advertising for organisations doing important work, and it seems to work.

However nauseating some of us find celebrity fundraising, they can raise awareness and money in a way you or I can't. If their celebrity can be put to some use for other people, whose conditions need financial support and awareness raising, is that not better than not doing it?
In the past few months, there has been some coverage of a young Scottish journalist who has just been diagnosed with MND, and he hopes to spend the short time he has left raising awareness and money for an MND charity. I was genuinely shocked to read how little funding and support is available for research and treatment / support. However eloquently he has written about his condition, it's an unfortunate fact that he will not have the reach or appeal that this kind of social media campaign has.
If I was in charge of the charity, or hospital ward, or community support service receiving the money, or a sufferer of the condition, I wouldn't be turning up my nose at it because of celebs' dubious motives or the naffness of the campaign. I'd just be grateful.