The facilities are yours and people use them
An arena, a round pen or stalls on a property you own are structures whose condition remains your responsibility. Somebody injured because of footing, fencing or a failing rail can make a claim against the owner regardless of who was riding or why.
A tenant boarding others changes everything
If a tenant takes horses for payment, there is a commercial equine operation running on your property. That is a materially different risk from a tenant keeping their own animals, it affects carrier appetite, and it is the kind of thing owners discover by accident. A lease clause requiring consent prevents it.
The structures need their own limits
Barns, stalls, arenas and fencing draw on the other structures limit, which is a percentage of the dwelling cover by default. On an equine property that default is almost always a fraction of what rebuilding would cost.