There is a big difference between content being "cut out" of the game and being sold separately.
"Cut out" implies that it was intended to be included, but was removed at last minute.
If Sleipnir was never intended to be included in the game (and there is no way to know that for sure unless you happen to have the game's design documents where this is stated), then it was not "cut out". Also, the content that it would drop from is not even in the game yet. There's no way to say that it won't ever be made available in game. It's unlikely, but possible, that purchasing it from the cash shop simply means you get early access to it. But if it isn't part of the content, then you can't say it was going to be part of it initially.
It's like how some products are labeled "batteries sold separately". Batteries are required in order to use said product, but it was not ever intended to be sold to you as part of the product's cost. In this case, "Sleipnir sold separately".
And sure, they could have reasonably (even if it did mean a delayed patch release) add in more content that offered the items as rewards. But then, that doesn't generate additional income does it? And additional income means that more content can be made available in patches (which is what we want, is it not?). If the purpose of the shop is to raise more money to support more/better servers and improvements to the game etc, then I have no problem with certain parts being "sold separately". I'd even be ok with optional dungeons* being sold, and I'd probably buy it. Why? Because I enjoy the raiding/dungeon side of the game, and why not balance the complaints with those who find their fun in minions/mounts/vanity.
*By this, I mean dungeons which are not required for roulettes or relic upgrades etc. Otherwise, it isn't truly optional. There's no reason why it can't drop the same items that come from equivalent dungeons, though. All this would do is simply increase my enjoyment of the game.