For one thing you don't need patent protections to keep 'low quality' vaccines out of the market. To my knowledge, that's what government regulatory bodies like the FDA are for.
> while otherwise making the license available at no cost?
Even if this was the case, I'm sure that Salk knew that if it was patented, that this would be only temporary (with no guarantees on reasonable pricing in the future); and it would be an unnecessary and immediate roadblock to helping people.