They cut to this flipped shot 3 times. It was done in order to obey the 180 degree line rule. In the scene, Jabba is facing from left to right, and Boushh is facing screen right to left. If they used the shot unflipped it would appear Fett was either behind Leia, or would almost appear as if Fett was drawing his gun on Jabba. When editing, Walter Murch suggests 6 things to consider in order of importance to making a cut. He calls it the rule of six. At the bottom of the list is 3 dimensional continuity, which is what I would consider this flipped frame. The editor made the right decision. Eye trace is listed fourth. I am willing to bet, that even among the hard core fans who saw this film in the theater in May 1983, 99% did not catch this flipped frame, and of those who did, it is a vanishingly small number remaining who were still bothered by it in any significant way. It is only now, after 30 years of repeated viewing, the advent of freeze frame and unbridled obsession over one's own screen faithful Fett costume down to the minutest of details that we find ourselves objecting to the shot and discussing it, myself included.
I would suggest that it would make a great experiment for someone to redo this sequence with the shot of Fett "fixed" to unflipped and see how it looks, and which would be more jarring to us now -- flipped or unflipped.