Non drivers benefit because workers at the businesses they patronize don't have to pay for parking, delivery people don't have to pay for parking, visitors don't have to pay for parking, etc.
That's an argument that's pretty hard to justify. Most zoning rules with parking minimums attempt to meet peak demand for free parking. Subsidized free parking means that all the extra spaces are paid for by the business regardless of whether they're used. So the business (and thus the customer) is always overpaying for parking, compared to some hypothetical business with no free parking, but reimburses all market-rate parking for visitors and employees. Of course, that hypothetical business could only exist in a dense or very pedestrianized area, and not all businesses are suited to zero-parking.
The point I'd make is that mandating minimums of free parking is absurd: businesses and developers could instead decide how much parking they anticipate their business would need. They could come up with many creative solutions somewhere between the extremes of zero parking and plentiful free parking depending on the area. In most towns and cities across America, except for a few dense urban cores, we mandate one extreme and the result is higher prices for everyone (including the non-drivers, who get to help pay for all that unused parking as well).
