Definitely worth a considered response, so I have followed up with our manufacturer, Keith. All the timber used in our Rustic Planters is recycled fencing material, some fences are made from hardwood, others from treated pine. I have noticed treated pine is being used more commonly than hard wood these days.
Generally the material is very old, that's why the fence was pulled down in the first place. That's not always the case however, it's not unheard of that fences will be pulled back down following a dispute of boundaries or when the fence is not to the property owner's specifications. Keith will use better quality treated pine in the base of planters in an effort to improve their longevity.
Don Burke offers this information on the subject of treated pine and vegetable gardens. In summary treated pine is safe for gardening.
Of course "safe" is never safe enough in some instances so we emphasize that the rustic planters are purely ornamental. When soil or potting mix is held in the container with direct contact to the timber the timber will quickly warp and crack so they are best lined with plastic or used for plunge planting potted plants.
Having said all that, yes we pot plenty of those D Handle baskets but they are very ornamental.