The Humane Society will check the premises for feed and may issue an order to get weight on the horses within a designated time frame. If the horses do not gain weight, the horses may then be seized and the owner charged. The HS often needs foster homes and if you push for it, will cover the feed costs and vet bills. They are powerless to do anything else until the judge makes a ruling.
This is absolutely right-some county humane societies (unfortunately mine too) only specialize in cat and dogs, do not have a sympathetic Prosecuting attorney, do not prosecute and have agents who can't even recognize a starving horse. However, the more people who call and complain about this, the more seriously they will take it. At the very least they usually put them on a 30 day inspection warning. They will stop in unannounced for 30 days to check for the 3 required basics: Food, water, shelter.
In our case, we call the Sheriff's department as they have a Mounted Deputy Unit and boy do they take it seriously! They will write up reports and they will prosecute. Either agency is going to always be looking for a foster home so that they can actually take the horses away.
I don't really know which way to advise you to go. We have 2 rescue horses at the barn that were aquired when people knocked on the doors of the neglectors and offered some money to "take them off your hands".
Whatever you do, don't offer to take care of these people's horses for them
. Buy them or foster them-you really don't want to become involved in the lives of these "trashlings".