I see your point, but I disagree. I think Temporal Investigations would prefer Picard's solution. Rasmussen intended to use his knowledge of the future to change the past. Allowing him to return would only cause greater harm. If upon further reserach Temporal Investigations deemed that Rasmussen was needed in the past to preserve the timeline, than they could wipe his memory and send him home. For Picard to simply allow him to return, though, would be disasterous."A Matter of Time" is a clear example of a temporal violation on Picard's part. He knowingly refused to allow a time travler to return to his own century and deprived him of any possible means of later doing so.