I have no such warning on Amazon, just add to basket.

Anyway, that would be hardly a surprise. All sellers sell exactly the same thing, they can not offer better GTA or better ESO or whatever, and they will not compete with price, not much room for it even if they did. They also sell over the internet (moreover sometimes digital copies), so they can not capitalize on location or distance, unlike classic shops. Generally, they have very limited options to differentiate from each other and offer some reasons to customers to shop in any particular e-shop.
Therefore, it only makes sense to offer disadvantages if a customer goes elsewhere. Okay, some prime, as they put it, membership may put off some, but it makes sure a customer who became a member to buy one thing will return if he needs another thing, because he will be unwilling to pay for second membership somewhere else. Because other e-shops will likely offer their own premium programs and promote them by putting desirable items behind premium-wall. Save for a few e-shops that will differentiate precisely by offering hassle free items, much in the same way GOG offers DRM free games.
In case of Amazon, move like that would be even more unsurprising, given they have been doing it for quite some time already with e-books and Kindle, coercing people buying e-books into buying Kindle (that would be that membership fee) and people buying Kindle into buying e-books on Amazon.