(OTOH, as ST VI showed, Spock was permitted to forcibly mind meld with Valeris to find out what *she* knew...so if a Vulcan can do it, why couldn't a Betazoid? Especially since Betazoids don't need to touch you to read your mind.)
As far as I know this isnít officially established as canon, but I got the distinct impression that a forcible mind meld is considered a reprehensible, almost unthinkable, thing for a Vulcan to do. However, as Finn pointed out, the stakes in this case were so high, and time so short, that Spock had no logical alternative.
Then again, trying to make logical sense out of anything in TUC is an exercise in futility. Ugh, what a frustrating film.