From the news article (found on the Oregonian's web site) :
The most serious risk is to children and gas-station attendants, a health official said.
Benzene gets into the air when vehicles run or when gas tanks are "topped off" that is, filled past the automatic shut-off point, the county said.
Though no evidence of the health effects is given, and thus no evaluation of the degree to which this is a serious health problem can be made, it seems a pretty strong argument against the prohibition on self-service gas. Benzine is a known carcinogen.