Keep in mind, a for-profit can have a social good mission and remain aligned to it (See Patagonia for instance).
Just because you are a for-profit does not mean you are beholden to pursuing profit at all costs. But yes, a B corp is another good option, though it has its pluses and minuses as well.
Perverse incentives. If a company's profit is linked to higher incarceration, they are incentivized to make the problem worse, not better.
This doesn't make all non-profits magical paragons of virtue, or all for-profits evil. It is simply an indicator.
I don't have that much faith in non-profits. But I don't have any faith in a for-profit company for social good.
Except there should be a much smaller supply of prisoners because many laws are unjust as they are now. The drug war directly serves the prison industrial complex and private prisons are one of the biggest lobbying groups against decriminalization and legalization.
Now maybe Promise won't lobby for the war on drugs because it would shatter their illusion of social good, but if they grow they will end up lobbying in other ways that will directly oppose public interests.