[Russ Noble]

The prime reason for mounting them horizontally as Chris P has said is for suspension tuning. If you look at your photo the joints are held between wide based brackets with long bolts, not ideal from a theoretical viewpoint but makes for easy and relatively quick handling adjustments merely by shimming the joints up or down within the brackets. Being able to adjust these also means you can change your ride height without compromising your suspension settings.

With a softer sprung road driven car you would most likely need more movement in the heim joint than a horizontal setup would offer particularly on the top link. Obviously binding of the heim joint could cause it to snap. With vertical mounting this is not likely to happen.

Cheers