It's possible to produce a report by user of poll actions, including poll.vote. In some cases (e.g. only a few users or only a few votes) this could compromise the anonymity of the poll, allowing users who can see sitestats to work out how someone voted.
It should be possible to label specific events as "not for individual reporting" in the system-wide config.
I'm guessing there may be other tools in future to which this would apply, such as the Questions & Answers tool under development, and Tests & Quizzes.