Yup, it's pretty much all about the manufacturer's warranty. If you buy a product from an unauthorized retailer, the manufacturer is under no obligation to honor their warranty. If something goes wrong with the product, you can still get it fixed at a repair shop, but you'd have to pay the full cost of the repair yourself. And if parts are unavailable, you're out of luck. The manufacturer has no obligation to offer a replacement model of any kind.
Now, given that many warranties are short (1 year is pretty common), the manufacturer's warranty isn't really worth much to begin with in a lot of cases. Add to that that you often have to ship products to a repair facility, and that the warranty doesn't cover the cost of shipping in many cases, and that warranty is worth even less!
But the problem with unauthorized retailers is that you really have no protection. They're shady to begin with. You don't necessarily know what you're buying. And if they screw you over, good luck trying to sue them! You've got no one to back you.
Plenty of online retailers are authorized. Most electronics manufacturers have a list of authorized retailers - both brick & mortar and online - somewhere on their website, or available if you call them. And while authorized retailers can only ever advertise the MAP (Minimum Advertised Price) as the lowest public price, they are completely free to sell for less if they want to. This is why you should always call on the phone for electronics, rather than just clicking "add to cart"
But my advice, stick to authorized retailers. If you're not sure if a certain website or store is authorized, call the manufacturer. And if you want a lower price than the MAP, call the retailer and haggle. "Grey Market", unauthorized retailers aren't worth it.