That's because he doesn't want you touching your truck, when you could be paying him to, lol.
In actuality, it's probably more of what I mentioned above. Waxes and coatings have very different hydrophobic qualities, and you're counteracting the highly hydrophobic coating with a wax that will actually help to hold water onto the surface of your vehicle. Notice how water beads up on wax, but runs off completely on the coating? That's the big difference a lot of people don't understand about coatings, and some are even deterred from coatings and think it "doesn't work" because they don't see the beading they are so used to associating with a good proper paint protection layer like wax. There's a big issue in detailing right now with products (and companies) that don't perform to peoples expectations, despite the fact that they are doing exactly what they are designed to do. To the point where some companies are putting out "ceramic" products that encourage beading, which is counter intuitive, just to make the customer happy and get the sales. It's an issue of educating the customer, vs just giving them what they want even though it's not the best for them or the vehicle.
And, as a detailer myself, the real concern is a customer using a product incorrectly and causing damage to work done previously that would need to be repaired or redone completely. If you apply some wax and put swirls in your coating, or even worse, your paint, he's gonna have a lot more work to do and you're gonna be unhappy with the coating protection, even though you caused the damage.
