Plausible deniability.
All the other evidence seals the deal.
Like Piquet's statement:
http://www.grandprix.com/ns/ns21798.html
Pat Symonds' non statement:
http://www.autosport.com/news/report.php/id/78647
paraphrased: "I don't remember, it was Piquet's idea, I don't remember, I won't answer....."
Flavor Flavio's idiotic "If I tell you to rob a bank, and you rob a bank, is it still my fault?" comment,
Plus the telemetry from Piquet's car, you don't go full throttle to prevent a spin on a F1 car. He claimed and telemetry confirmed full throttle up to the point he hit the wall.
Add to that the fact that Alonso's fuel strategy made no sense. There is no reason to start as light as he did as far back as he was, on a track where passing was all but impossible.
As screwed over as he was, I am still surprised Piquet came forward. With the grid expanding next year, there should be some open seats for an experienced driver with a superlicense. I can't see anyone giving him a ride now.