The eth_sendTransaction JSON-RPC method is not supported because Infura doesn't store the user's private key required to sign the transaction. Use eth_sendRawTransaction instead.
In the Web3 tutorial, we use web3.eth.sendTransaction, which in turn signs the transaction locally using the private key of the account, and sends the transaction via web3.eth.sendSignedTransaction, which is a wrapper for eth_sendRawTransaction.