This whole "luxury tax" thing gets tiresome. All it really means is, "I'd rather pay a lower price." As with any product, either the price seems fair to you, or it doesn't. Some people will think it's fair, others won't.
The price of factory-installed storage, regardless of manufacturer, is nearly always higher than after-market storage parts. What people conveniently ignore is that there's a cost to adding/reconfiguring storage - the time it takes to shop for it, the time it takes to install it, the time to configure it (if formatting, restoring from backup, etc. is necessary). In other words, you're comparing the parts-only cost of a do-it-yourself project to the "convenience price" of fully-installed and factory-guaranteed capacity. A do-it-yourselfer has the luxury of making that price comparison (and charging themselves zero for the labor - after all, they may consider it to be fun). For anyone else, whether it's a business that has to pay its labor costs, or an individual who would bring their gear to a shop for the service... after market can be more expensive than buying factory-installed.
There's no such thing as a free lunch. You want 500GB of "free" cloud storage to go along with your new 1TB Mac? Expect to have that cost built into the product's price. And for something clearly more costly to the manufacturer, just how long should you be able to use that 500GB of storage at no charge? For as long as you keep the machine running? Or will you happily start paying for it after a certain amount of time passes?