I didn't get a survey invitation for over a year, and when I finally emailed them about it a few days ago, I got the following message:
"e-Rewards Member Services to me
show details Sep 24 (1 day ago)

Thank you for contacting e-Rewards. You inquired regarding the decreased frequency of survey invitations that you receive. We have reviewed your past participation in the e-Rewards program and have determined that we stopped sending survey invitations to you as a matter of policy.

e-Rewards serves clients in the consumer packaged goods industry, auto industry, and in general to companies who provide products or services that are sold to businesses and consumers around the world. These clients use survey results to make decisions that have significant financial implications for their companies. As such they rely upon the quality of the survey answers they receive from e-Rewards panelists.

From time to time our clients report that a specific panelist's answers to a survey are unusable because the e-Rewards panelist clearly did not provide consistent, plausible answers to survey questions. When we receive a number of such reports about the same e-Rewards panelist, we stop sending survey invitations to the panelist, as panelists who do not provide honest and thoughtful opinions are not of interest to our clients.

Panelists who are no longer invited to participate in surveys may still have use of their e-Rewards accounts, and may redeem any and all outstanding balances at their disposal.

Sincerely,
Member Services"

Wow, so they do this, after all!