Pew Research does not use volunteers for their polls. They state in their frequently asked questions that “a survey of volunteers is a ‘non-probability sample’ and the results cannot be generalized to the public as a whole.”
The key to research conducted by a survey is it needs to have a random sample of people so that every type of person has an equal opportunity to have their views captured in the survey. If the polls accepted volunteers, it would violate this principle because not everyone would have an opportunity to be included.
However, people should participate in research surveys to express their opinions to the leaders of the nation. Public officials and other leaders pay attention to the results of polls and generally take the result into consideration when making decisions.
Polls or surveys measure public opinion and document the experiences of the public on a variety of topics. The results of these surveys provide information to be used by those in academics, researchers, and government officials. The polls help keep leaders informed so they can make wise decisions that benefit the people as a whole.
Major professional organizations of survey researchers have a clear code of ethics for their members. The codes cover the responsibilities of pollsters with respect to the treatment of those who respond, their relationships with clients, and their responsibilities to the public when they are reporting on polls.