It isn't McDonald's, or any other place that you can order anything they sell. Have you seen the inside where the window is handled? It is in the pharmacy where all the PRESCRIPTION medicine is kept.

Therefore it stands to reason that the drive through is for prescription medicine only.

I can see you may not have known this ahead of time, and I can see how it would have been nice if they made an exception. But they didn't. Once told I can't see why it is hard to understand. I don't think they need a sign.