There is also a difference in e.g. stopping random people on the street and asking if they are willing to participate, or posting an ad online and waiting for people to call, even though both are voluntary.

>I would say that most surveys/studies are done on a voluntary basis.

Also most surveys/studies carefully say that they are not necessarily representative.

Yes, a disclaimer over the validity of the results would be nice ;). But to be fair, most surveys are taken rather seriously, especially by the common populace and the main stream media, ignoring the lack of absoluteness in the results.