So you’re going to test me and not tell me the results despite the fact I might kill a vulnerable member of my family if I come into contact with them? Yeah go fuck yourself
Yeah that is unfortunate, but playing devils advocate, the alternative is no different. Here you may help research efforts to mitigate the disease.
What's the point of not telling you if you tested pos or neg? Are they afraid that with your responsible behavior you can spoil the results by not infecting more people? Are they studying how widespread it is at a given point in time, or are they studying how it is spreading?
I'd guess that if they tell you the result, they'll bias their volunteer pool towards those who want to know if they're sick - presumably because they've got symptoms - rather than getting a less biased sampling more representative of the average population.
They can't tell you your results because you're anonymised.