Because when I'm browsing Twitter my first thought is usually "Gee I wish I could buy some random thing."
OcculusVR: New Rift is out! [Buy]
SXSW: Tickets on sale now! [Buy]
JKRowling: Just release my new Book [Buy]
Steam: [Some new Triple A game] is released [Buy]
RaspberryPi: New Pi Beowulf cluster available for purchase [Buy]
RedCross: [Some disaster] we need your support [Donate]
(I'm sure a donate will come along if this works)
Anywho, there have to be _tons_ of impulse buyers in the world. People will click
That would be a very relevant functionality : Twitter is all instant emotion and therefore a perfect platform for impulse donations !