Well, you are supposed to know who he is by then. In fact, you're supposed to know and hate him pretty damn well by the time you get to ESB. Seeing him there is not meant as an introduction, but to show the emperor for the first time since ROTS. And to respond to what theredbaron said, I just don't like inconsistency. As an example, I also saw the TPM puppett Yoda first, but I consider the Yoda from AOTC and ROTS way better, and more suitable for all the films, even though I saw TPM, ANH, ESB, and ROTJ Yoda before I saw the CG Yoda.