Walmart isn't happy that Visa still allows customers to sign for purchases made with their chip-equipped debit cards. The retail giant has filed a lawsuit against Visa in New York in an effort to compel the credit card brand to require PIN verification when paying in its stores. Walmart argues that PINs are a lot more secure than signatures and can help prevent fraud. It used to only allow debit card payments verified by PINs when it first started accepting chip cards, but Visa forced the company to allow signature verifications.