I'm not sure if you understand what MacMillan is about, but you're comparing apples and oranges here.
"Fuck Cancer" is all well and good... but MacMillan's work isn't about finding a cure, it's about easing suffering. They provide support and care to patients and their families, "Not Alone" sums them up pretty perfectly.
Compare these two adverts -
"Not Alone", MacMillan Cancer Support (http://www.youtube.com/watch?v=MLpS97XA5VA)
"Cancer, We're Coming For You", Race for Life (http://www.youtube.com/watch?v=GWJEmMDQXoA)
Yes, fuck cancer.