SPENDING $3.3 billion on an unprofitable business might seem an undisciplined splurge. By buying Jet.com, a shopping website, Walmart, the world’s biggest retailer, has joined the ranks of investors betting on so-called “unicorns”, or private startups valued at over $1 billion. The acquisition is the most expensive deal ever for an American e-commerce startup, and a sign of just how worried Walmart executives are by the rise of Amazon.