The idea of public investment in public goods, like transit, makes no sense to capitalists who intend to profit from the private for profit provision of transportation as a private good.

You misunderstand the criticism.

Making buses free is the opposite of public investment in transit. It's disinvestment. The NYC bus system collects just under a billion dollars a year in bus fares. If you eliminate the bus fares, you are slashing the transit budget by a billion dollars.

Now, you're slashing the transit budget by a billion dollars for a cause that progressives find symbolically nice - but it's still slashing the transit budget by a billion dollars. And considering that the total NYC transit budget is just under $20 billion, and the bus portion of that is only about $6-7 billion, that is a huge reduction in the funds available for running buses.

So this is terrible for bus service. And for NYC transit service in general. The customers get more money in their pockets, but the transit agency now has significantly less money for operations. Which means they have to cut service more than they otherwise would. Worse, those cuts have to come at a time when usage will rise more than it otherwise would have - because now it's free. And you end up with non-destination riders who get on the bus just to have somewhere to go. So you'll have more riders, and fewer resources to provide them service.

So the system degrades for everyone. It's free, but it's worse.