Note: I'm also a Texan who hasn't decreased his Amazon spend since sales tax started, if anything I've increased my Amazon spend over the last few quarters.
This way they can build local fulfillment centers (which force them to collect sales tax), and yet not actually have to charge it.
It doesn't work for every item, but it does for quite a few of them.
That's very different from the potential profit profile if Amazon were seeking to maximize near-term profit as opposed to maximizing near-term growth, with the expectation that that maximizes long-term profit.
It merely has no obligation in most jurisdictions to collect sales tax on behalf of the local taxing jurisdiction. Those purchases aren't tax-exempt; it's just that it's the buyer's problem to comply, not Amazon's problem to comply. That many buyers "forget" to comply does not make them exempt. (And Amazon is not exempt, rather they never had an obligation in the first place.)