Amazon knows if a product has been bought with a discount and the amount of the discount; it can outright ban reviews from people who bought with a discount, or put these reviews up for special "review" (either human or automated).
You can still leave a review when you didn't buy the product via Amazon (or at all), so in theory vendors could send products outside of Amazon to get reviews. This however would become very complicated, and Amazon could still spot and review reviews that don't follow an Amazon purchase.
The only option left for vendors would be to ask reviewers to buy the product on Amazon for the full retail price, and then refund them outside of Amazon; not impossible, but difficult.
This change may be good in theory, but it will make it very hard for new products to exist at all / rise above existing ones, esp. given the way Amazon search engine (A9) apparently works -- ie, strongly favoring products that already sell a lot, even if they don't quite match the search keywords.
One could think of another option, which would be to offer users the possibility to filter out all reviews that don't come from a "verified purchase" (incentivized reviews aren't "verified purchase"), but still let them exist.