The explanation for that scene is provided in one of Aaron Allston's X-wing books. It's delightful. One of the Wraith Squadron operators is a slicer, and he describes the reason he feels so guilty. Paraphrased: "I was at the Battle of Endor, and as we watched the Death Star blow up we knew that the Emperor had died. I sliced into the planetwide newsfeeds on Coruscant and looped the footage of the Death Star's destruction again and again. The people in Invisec (the sector of the planet where the unwanted and politically dangerous are kept) rose up in a riot of celebration, setting off fireworks, tearing down statues. And then the stormtroopers came."

That's the EU at its best: It takes one of the off-kilter scenes from the G-canon, one of the scenes that offends a rational person's sensibilities, and recontextualizes it in a way that makes sense. The celebrations on Coruscant were not planetwide; it was just a small group of ghettoized malcontents. And their impudence was not tolerated.