Don't feel too done by regarding not using chairs. The rouges do it to cut time and cost, not that it's much because I'd think most people would be prepared to pay an extra $200 to have it done properly. Drilling into the slab is called doweling and is an accepted practice. It's especially good because you eliminate the chance of either the slab lifting or sinking causing a step between two surfaces.
The whole issue with this is, and it's no fault of the owner, is owners accepting this method.
Unless you know how concrete should be laid then how are you to know that without 25mm concrete cover it's guaranteed failure of the reo especially when laid directly onto soil.
Why do we put reo in concrete? Because concrete has incredible compressive strength but low tensile strength.
Why do we put reo on the bottom "generally"? Because when you apply a weight to a slab, the bottom stretches (where reo acts as the strength) and the top compresses (where concrete has its strength)
So that's how the two generally marry together.
How it fails when reo is on the bottom is it corrodes and expands the concrete causing cracking.
Sorry for the long post.