In Back to the Future, when Marty is stuck in the past, he visits a known diner from his timeline and asks the server about the phone booth and the address of Doc Brown. The server angrily asks him "if he's going to order something", to which Marty asks for a "Pepsi free" and the server replies "If you wanna Pepsi, you're gonna pay for it."
Why and how was Pepsi free in Marty's own time (i.e. 1985)?
Was Pepsi free during that time in the real-life USA?