I guess they're being laid on ground where the turf has been stripped?
If a good hardcore base (at least 100mm (4"), but 150mm (6") is preferable) is spread and compacted, that would be considered a much better base.
On top of that, concrete could be laid, and the slabs bedded on with a few dabs of mortar.
What's often done is to just use 2" of compacted sand instead of the concrete. This is ruled off straight, and the slabs placed directly on to it.
There are other favoured methods, but simply laying mortar on the topsoil/subs-soil would not last long. Traffic, heavy or light would not matter. The self-weight of the slabs plus natural settlement would result in gradually sinking, creating a far from level finish.
A patio, as with anything else, is only as good as its base Colclough :o)