I saw someone on another site come up with an explanation for Spock's behavior in this episode. This person suggested that Vulcans always have some tenuous telepathic awareness of other Vulcans. Nothing like a full mind-meld, of course, just a tenuous sense that they exist, with a bit of their general state filtering through.
If this is so, then during the 23rd Century, each individual Vulcan is partly restrained by the collective restraint of all other Vulcans.
No one Vulcan has that much effect, but the cumulative effect of a planetful of billions of them could be fairly powerful. During Spock's time in the past, then, his normal personal restraint isn't enough to overcome the telepathic effects of an entire planetful of UNrestrained, savage Vulcans.
We know that Spock felt it when the 400 Vulcans on the Intrepid
died (during "The Immunity Syndrome"), so we know that he has some telepathic awareness of other Vulcans at a distance under some circumstances. Whether he has any telepathic awareness of other Vulcans when they're not dying is something TOS doesn't make clear.
As I said, this isn't my theory; I just read it somewhere. But I thought it was reasonably plausible, and I thought it took an explanation for Spock's behavior in this episode that seemed really stupid on the surface and turned it into something really interesting. If this person's theory were true, then going against Vulcan norms wouldn't be as easy as just deciding to do so, because there'd be all that telepathic influence. Something neat to think about.
Of course, the fact that we get ANY explanation for Spock's out-of-character behavior in this episode is due to Mr. Nimoy's diligence. When he got the script, there wasn't any explanation at all, and when he asked the third-season producer why, Fred Freiberger hadn't realized that Spock wasn't supposed to do such things.
The mind boggles! Nimoy told Freiberger that he couldn't play Spock with the script as written, Freiberger said there wasn't time to rewrite it, and they eventually compromised on the hand-waving explanation we got. It sucked as an explanation (unless the theory above is correct), but it was way better than no explanation at all.
Why Roddenberry insisted on bringing Freiberger in as the third-season producer, instead of promoting the quite-deserving Bob Justman to lead producer is just one of the mysteries.