Costco is free to pay their employees as high of wages as they'd like, no legislation required. This lobbying isn't out of their good will but a strategic play to hurt competitors that rely on lower wages. Such is politics.
If Costco already starts all employees at $11.50, increasing the minimum wage to any point less than $11.50 doesn't initially impact their own bottom line, but just serves to increase their competitor's costs.
http://www.nytimes.com/2005/07/17/business/yourmoney/17costc...
There's no reason why corporate interests and good will can't sometimes go hand in hand.