1) Voting against Boehner, even voting present, would have made him a freshman congressman voting against the most powerful congressman in the House. Say what you will about Boehner's votes and tendency towards RINOism, but you cannot honestly say that he doesn't wield immense power in DC.
2) There were no viable options to vote for. The Speaker position is not about who is most conservative, it is about being able to plot a course for the House caucus and being the public symbol of that message. I agree with many conservatives that Rep Boehner has a terrible track record for representing the conservative position, but no one else made a good case for why they would be a better speaker. Several candidates rose up under the 'anyone but Boehner' banner, but up until just days before the vote, no Republican congressman had the courage to step up and announce they were running for speaker.
3) Rep Emmer was not elected by only Tea Party Republicans in the 6th CD, nor does he only represent them. He was elected by and represents every person in CD6. The majority of Americans took no notice of the vote in the House, and even in the conservative 6th district, Rep Emmer's vote is likely already forgotten by the vast majority of the people who voted for him.
If there is an example of Emmer saying bluntly and directly that he would not vote for Speaker Boehner, then that video/audio should be brought forward to let the congressman explain his vote. But over 24 hours later, no such evidence has appeared, and no activists have stepped forward to say "Tom promised me he would never vote for Boehner". What seems much more likely is that conservatives who supported Emmer projected their own views on Emmer the candidate, and are now amazed that he isn't following their own unspoken wishes.
And lastly, there is a sense of irony that Minnesota conservatives are so loudly complaining about one of only three Minnesota congressmen who did NOT vote for Nancy Pelosi. There are legions of conservatives in CD 7, 8 and 1 that would give their right arm for a congressman who would even listen to them, let alone vote along conservative principles.