At the end of "A Matter of Time," purported historian Berlinghoff Rasmussen is held on the Enterprise after it's been determined that he lied to the crew and attempted to steal technology from the ship.
I can understand Picard being annoyed with Rasmussen for interfering (sort of) with the mission, and certainly being peeved by the attempted theft of various pieces of technology. However, is satisfying his anger as important as allowing the timeline to potentially be violated? The timeline could change as a result of Rasmussen failing to report back to his time period. Perhaps Picard, Riker, Kes, or one of Dr. Crusher's plants would never be born as a result. (Probably very unlikely, but possible.)