it wasn't just a survey. it was 'everyone get tested- if a certain percentage of you (or more) fail the PED test, we will enact penalties. and regardless of what the pa says, enough players failed the test to start the penalizing phase. the union is ****** (rightfully so) that the names got matched to the samples and were made public (some of them).
if it was just a survey and no testing, every player would just say 'no, i'm not juicing' and that would be the end of it. sadly for the pa, taking players at their word would not be enough for the issue to go away, hence the tests.
at the time, mlb and the pa were denying that roids were a big issue. the players thought 'im not gonna stop juicing cause nobody is supposed to find out its me', so they took the tests and failed