That's not how the game goes.. AT&T doesn't really care about where the 600 dollars plus the cost of line access and 2 year commitment comes from. To them its a sale. Beyond that, why should AT&T pay to stock Lumia and then have a floor salesperson tell buyers to leave them on the shelf.
If they worked for me and I bought 50 thousand dollars worth of product and you tell people not to buy I'd make sure you'd never work at any AT&T store in the state.
It's not about AT&T caring where the revenue comes from, it's about maximizing revenue; which sometimes depends on the relationship they have with the various OEMs or local distributors. If there is an additional incentive coming to the store, or to the salesperson for selling a given product, it is in their financial interest to push it. If there are more aftermarket accessories for an item which could result in additional sales, they will often push those products. Even if the salesperson does not discourage sales, when I walk by an AT&T store, almost all the advertising I see is directed at the iPhone and Galaxy lines. I'll look again, but can't remember any Lumia advertising. Ironically, the biggest single ad on this page is for the Galaxy S7 Edge.