However, that belief isn't totally without basis. Facebook fired its entire trending topics curation team [1], after allegations that its members suppressed conservative stories [2]. Even though Facebook denied that any ideological suppression took place, their actions in firing the team could be interpreted as confirming them, especially considering how badly their algorithm performed without human oversight afterwards.
[1] https://www.theguardian.com/technology/2016/aug/29/facebook-...
[2] https://gizmodo.com/former-facebook-workers-we-routinely-sup...
Restricting the platform is their right. But the issue is people believing they are being censored and by banning certain ads they reenforce that viewpoint.
To me it looks like the polls say that they believe these companies censor content, and that these companies also lean left. Nothing about their viewpoints.