Boarding horses are just that feed, water, bedding if stalled, or good fences if they are pastured with a shelter and plenty of pasture & water.
That is it.
Vet care, farrier, training, etc. is up to the horse owner, unless you agreed to other things at the time you had them sign a boarding contract.
I agree with the other posts as to them traveling,must have money to travel, so they will have to travel less and take care of their responsibilty with their horses.
Sometimes boarders will take advantage of your kindness, do not let that happen.