It is a fact that rules were first introduced so people could play battles with the numbers of figures they had in their collections. The game helps sell miniatures in a lot of ways, and to others the rules don't matter only the figures (which is the majority).
'But if that's the case, why is the demo sales process all about putting the idea of a game into their heads?'
It is not all about a demo game, it is about offering the game, as well as the modelling side. You are making assumptions again. Some people lap up the game, some don't like the idea and instead want to model/paint and are given a short lesson in that instead, others want both and some won't want either.
The game itself is the biggest sales opportunity in most cases as it immerses you far more in the product/universes than purely the painting alone. It also encourages more sales in general to participate in that side of the hobby rather than just building a box or blister of miniatures and painting them up.
I have worked for the company, and therefore have an 'educated viewpoint'. I no longer work for them and have no alleigance to them, and I especially don't like their pricing scheme.
Again, the game is not the be all and end all, it opens up an avenue for profit but I know if the models were dire people wouldn't collect them most of the time. There are again exceptions to this such as tounament gamers etc who 'need' certain models/squads/units to win their games. The tournament scene is also comparitively small compared to the 'hobby' scene.