U.S. Bancorp and Wells Fargo & Co., two of the big banks behind person-to-person payment system Zelle, said this week they're committed to keeping their customers' accounts secure after a report of scams aimed at Zelle users. Zelle lets users easily transfer money to other people via a smartphone app. The service, launched by Early Warning Systems, a joint venture of a half-dozen of the biggest names in banking, is growing quickly.