Say you have nothing planned one Sunday, and then someone offers you a free trip to the center of London, T-Shirt included; who wouldn't join a student protest against higher tuition fees?!
This student in particular feels very strongly for the cause.
(No, I don't have a camera phone, but my wide angle lens put him outside the cameras aim, but well in the picture)
3 months later, some old lady in France wonders why she has a thousand flat balloons in her backyard.
One last thing must be said: Protests are much bigger in America.