there is no federal maximum mandate, for
hydrocarbon content, any fuel distribution
operation can sell 100% hydrocarbon, if they want to.
the mandate, is for the minimum content
of the product of the refineries.
In other words, you buy from the refineries,
or else. how nice of them
Or else you don't buy any gasoline because all gasoline is manufactured in petroleum fuels refineries.
The EPA exempts "small refineries" from its mandate because the mandate would be overwhelmingly uneconomic for them. Exempt refineries comprise a very small fraction of US refinery capacity.
Sure, one can still retail 100% hydrocarbon motor gasoline, if you can get it. It's generally available only in areas served by "small refineries" that serve relatively small markets.