But the truth is, the crew broke the Prime Directive the moment they decided to screw around with the volcano at all.
And Kirk got chewed out and demoted because of it. Seems pretty consistent with canon to me.
As for Spock's attitude, in TOS Spock didn't follow a strict and rigid definition of the Prime Directive, and showed a willingness to bend it on occasion, or at least go along with Kirk when he chose to interpret the Directive his way. Obviously saving the tribe from a volcano is deemed an acceptable bending to Spock, provided it can be done without the natives being aware of it.