According to Alejandro Zelaya, businesses will not be penalized if they do not accept bitcoin.
The rating agency said that the law and other policy decisions had “weakened governance” and increased tensions with other countries.
The bank points to bitcoin's illiquid nature, volatility and U.S. dollar conversion risk as major limitations for its use as legal tender.
The poll, which took the results from 1,233 people between July 1 and July 4, also showed 46% knew "nothing" about bitcoin.
President Nayib Bukele made the announcement during a national address on Thursday.
One plaintiff says El Salvador's president failed to consider the law's harmful effects.
A bill currently before the California Senate has been amended to include a provisional application process for early-stage bitcoin startups.
As bitcoin continues to be taken more seriously by banks and regulators, the industry needs lawyers to help smooth out bumps on its way to mainstream.