In this case, everyone who bought a Mac Pro knew that they weren't going to have 20 cards to choose from 6 months down the line. However, this computer was advertised as the most expandable Mac ever. That means you can upgrade more than the hard drive or RAM, which you can do with any Mac. It should mean that after 18 months there is at least one more option as far as video cards are concerned. Apple store employees were making sales while telling customers that there would be new video cards at some point in the near future.
You're making an assumption. When I bought my B&W G3, 18 months later, Apple didn't have an upgraded video card available for me to buy. When I bought my G4 towers, it was ATI who offered an after market solutions. None were offered from Apple. The fact that some cards worked in older machines were just coincidence and Apple never marketed those cards to owners of older models.
Which ever Apple sales rep made those statements would likely get reprimanded if Apple ever found out because it just isn't true. Apple has never done this.