Starbucks' CEO said infrequent customers are becoming more cautious spenders.Jeffrey Greenberg/Universal Images Group via Getty ImagesUS consumers are tightening their purchases, Amazon and Starbucks executives said Tuesday.Amazon CFO Brian Olsavsky said their customers were trading down and looking for deals.Starbucks CEO Laxman Narasimhan said the coffee chain's customers were growing more cautious.US consumers are becoming more prudent with their spending, executives at Amazon and Starbucks said on Tuesday."Customers in the US are being very thoughtful about their spend.