SurveyMonkey is just trying to protect their brand, which is fair, while PopSurvey seems to be making a bigger deal about it than they should - they tried to market with monkies for obvious reasons, and now they are trying to look like they are getting pushed around.
I don't know enough about the law and trademark and parody to know comment on the legal aspects of this, but I have to say that I actually appreciated the tone of SurveyMonkey's attorney's letter.