This is a "scam"...not a scam in the literal sense of the word to but Horizon Hobby scamming their customers. In other words, it's a marketing ploy, cooked up by company executives, for no other purpose than to make more money. I'm not saying that making money is a 'bad' thing...after all, to companies are in business to make money. However, when companies make decisions purely for the purpose of additional profit, and having complete disregard for their customers, that's something I will NEVER agree with.
I'm willing to bet that, before the SCX10-III was ever released...heck, before the build manual was ever written...that the straight axles were already ready. They probably developed them alongside the portal axles...but, when the marketing team & company executives got involved, they realized they could make more money by selling the kits & RTRs with the portal axles, and then, at a later date, offer the straight axle "upgrade" kits.
Think about it - which would make them more money? Selling kits & RTRs with the straight axles, and then offering a portal axle upgrade kit at a later date? Or, selling kits & RTRs with the portal axles, and then offering a straight axle upgrade kit at a later date? The OBVIOUS answer is the second option, as they would have known more people would prefer the straight axles, thus more people would be willing to spend additional money buying the straight axle "upgrade" kit. Such a shame...and a sham.
~ More peace, love, laughter, & kindness would make the world a MUCH better place