The gross violation of the PD is a biggy though. On that day, those people were meant to die in a volcano, by altering those events the crew have chaged fate. They decided to play god and have no clue what their actions will result in. The PD isn't just a suggestion for behaviour, it is Regulation #1 for all Starfleet personnel.
My God! This is such non-sense.
Doctors change fate every single day. If you contract cancer, should they just turn their back on you and tell you you're fated to die? I have an incredibly tough time respecting a person who suggests its better to allow an entire species to die instead of helping them.
At the end of the day, Kirk's interference will have little or no impact on the Niburu. As the species evolves, the encounter will be chalked up to native superstition. Just like it would've simply been better to beam Palmer up in "Who Watches the Watchers" and allow Leto to believe in the "Picard". Stories would be told but would've fallen out of the spotlight as the culture evolved.
The Prime Directive had never, ever been used as an excuse to watch entire species die until TNG.