The farm subsidies are to ensure enough farms are profitable enough to have an excess in the event we lose a ton of farmland or crop. If something happened we would not bounce back fast enough if we did not have excess, and excess would not happen if the free market pushed prices so high or low that the change in demand caused people to stop farming.

On the subject of bus fares, it's not that simple. If bus fares increase, some people will stop riding the bus and switch to cars or other forms of transport (or not go at all) which will likely reduce fares and possibly increase congestion which would slow down the busses which would cause less people to ride them which would...

I'm not saying that's exactly what would happen, I'm just saying you have to be careful. People only ride the bus when it's more affordable or more convenient