I think the reasoning is that people who need affordable housing are unlikely to be able to afford a car and the parking for it, so they have to live near publi

I think the reasoning is that people who need affordable housing are unlikely to be able to afford a car and the parking for it, so they have to live near public transportation. If the people in the affordable units can't afford cars, it doesn't make sense to require developers to build parking for them. The foot traffic from public transportation makes areas near public transpo appealing to businesses. Then the area becomes "hip" and affluent people want to live there to be close to the businesses. They don't want to use the public transportation, they just want to be close to where the restaurants/bars/shops are. Then prices in the area rise, pushing out the poor. So now the poor, who have to take public transportation, get pushed away from public transportation. You could try to fix that by building public transportation to the areas where the less affluent live now, but eventually the same cycle will likely happen to that area as well. Forcing units to be affordable if you don't build parking for them forces developers to rent to people who have to live near public transportation. Otherwise, the units would be full of affluent people who don't drive, but also don't, or at least don't have to, take public transportation. If you push the affluent out, they'll buy cars to get back to where the stuff is. If you push the poor out, their backs are against the wall; there isn't a fallback plan.