Not so, at least among "legacy" U.S. carriers. The real reason is that purchase patterns of "bottom feeders" are far more susceptible to this sort of influence than those of pax who spend more. Tying points/miles to spending would reward people who statistically* don't much care, while missing an opportunity to score big with folks who do. Discount economy tickets may not be as profitable as full Y or paid F, but they're a heck of a lot more profitable than empty seats.

VA may think their market is non-traditional in this regard, they may be influenced by European FF program patterns (which originated in a market where airline choice is influenced by different factors, so motivating pax who have already chosen an airline to pay more is a major goal), they may figure it's worth a shot, or they may be making a mistake. Time will tell.

There are many reward programs outside the airline business whose rewards are tied to spending. (I'm personally in those of Staples and REI, to name just two.) Taxability of those rewards has never been an issue. The tax issue with airline FF awards has to do with separating out how much of the fare is for transportation versus credits toward an award and issues along those lines, not with how credit toward awards is earned or calculated.

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*FTers are not typical. Repeat: FTers are not typical.