The reason that you were refused is because walk-ups at a drive-thru teller (or any drive-thru) are a SECURITY RISK! You might be harmless, but the guy that attempts it after you just might be a bank robber.

You were refused in order to protect the safety of the bank employees. This is standard procedure at every single bank across the U.S. BofA does it, Chase does it, Wells Fargo does it, Fifth Third and so on. Even the little small town banks do it.

No sign should be necessary.