Presumably, the CDC is trying to avoid causing mass hysteria. Unfortunately, people at high risk of having already contracted the disease will continue to go about their lives when, perhaps, they should be under closer observation.
It's obvious that the CDC is aware of the elevated risk of transmission. Why else would they be seeking out 130+ passengers from that flight the nurse took - when supposedly there was no chance of passing the disease before 'symptoms' appeared.
I don't buy what they are telling the public about symptoms needing to be present before the virus can be that transmission - at least to the degree that they are reporting.