Whose idea?

Stephen Colbert's comment: "The launch was to demonstrate that the Falcon was not only reusable, but the Falcon Heavy can carry a large payload. So to test it, Elon Musk sent up his own Tesla convertible with a dummy astronaut at the wheel, blasting David Bowie's 'Life on Mars.' That's right, this is absolutely true—a giant phallus cranking rock 'n' roll, releasing a red convertible into the dark void. And the award for Most Midlife Crisis goes to Elon Musk."

I think it was pretty genius marketing. On a first test like this, nobody is going to put a multi-million dollar satellite in it; and definitely not a manned flight. Before the launch, Elon himself gave the launch a 50/50 shot at success. They needed something with mass, so why not take advantage of the opportunity?