Wow, great point! Except for two things; first, if people with strong political beliefs were more likely to reply to surveys, would that mean that basically every political survey was inherently biased? Second, if you look under the section of Appendix A marked, "incentives," you can see that they corrected for sampling bias by offering higher incentives to groups that have a lower response rate:

All respondents were offered a post-paid incentive for their participation. Respondents could choose to receive the post-paid incentive in the form of a check or a gift code to Amazon.com or could choose to decline the incentive. Incentive amounts ranged from $5 to $20 depending on whether the respondent belongs to a part of the population that is harder or easier to reach. Differential incentive amounts were designed to increase panel survey participation among groups that traditionally have low survey response propensities.

Anyway, do want to keep trying to prove that the Pew Research Center doesn't know how to conduct a survey, or are you finally tired of digging?