Well, in the 60's, wasn't it generally the fashion to have a shaved chest on TV? There were some exceptions, but most often than not, guys were hairless (I don't even know of we saw a lot of armpit hair - NEVER back hair). The actors who did have a lot of hair poking through the tops of their shirts never seemed to have bare-chested scenes. I remember this one episode of The Fugitive where Richard Kimble opens his shirt to show someone some bruises. David Janssen had a thick mat of dark chest hair - and tellingly his back was to the camera. You could argue they didn't want to show any unsightly wounds and left it to the imagination, but in beefcake photos of the actor he has no chest hair. SO it seems to be more like a "I'm not gonna shave for one shot" and he had a TON of hair.
Why didn't Nimoy have to shave? He probably didn't want to if he was only going to have a single scene without a shirt (why put up with the itch). He wasn't going to be the "shirtless hero" like Shatner and his puffy nipples. So they probably justified it as his being an alien.
Nowadays, a lot of action, beefcake stars go hairless to accentuate muscular definition (Chris Evans in Captain America, Henry Clavill in Immortals). Also, the general consensus today is that hair is yucky (should we even get into the female bikini shaving area?). Chest hair seemed to be a sticking point: it was dirty or animalistic - Sean Connery kept his for Bond, but Roger Moore had none. And it showed in their characters.
Wow, off on a tangent here…