I don't know if that ranks as an all-time robbery. The first round was very close. The second round was clearly and undisputedly Pearson's, so the judge scoring it 30-27 Sanchez should be criminally investigated, as well as banned from judging main card fights.
The third round, for the first three minutes, was slightly in favor of Sanchez, and then he got overaggressive trying to seal the deal and Pearson won that round going away. For people claiming Sanchez did nothing, that's not true. While the announcers were talking about how Pearson was slipping all the punches, in the third, Sanchez hit him on the button and staggered him, for instance.
Any time you get a fighter who stands in the pocket and slips punches, there's a good chance that the other guy, especially if he's the crowd favorite, and/or if the other guy is winging with more volume, will get credit he doesn't deserve. Happened to Marvin Hagler when he fought Leonard, for instance. Happened recently to Lawler against Hendricks when people acted like Hendricks was doing good damage in the opening couple of rounds.
That style/skillset makes for some weird scoring.
So, while I completely disagree with the decision, I don't think it's THAT miserable of a decision.