“PLEASE NOTE: A valid Diablo IV key linked to your Battle.net account is required to post in this category. Additionally, users who troll, harass, or try to incite nonconstructive discourse will be suspended from this forum.”
Is this statement from Blizzard correct or can anyone post on this forum. Are people who say that they will not purchase the game liars because they already pre ordered but are pretending that they will not purchase?
Not sure. Maybe they didn’t turn that on yet because the game hasn’t launched.
Nobody likes a snitch 
Only a chicken sandwich was required to get an early open beta key. That key still being valid is why you see all the posts. I would expect that will continue unless Blizzard changes rheir mind and invalidates those keys.
I haven’t bought the game and I can post here.
But the first time I checked the forums I couldn’t.
The limitations were for the first round of Beta and will be true for Release. They removed the restrictions for Open Beta weekend. I don’t think they have re-instated them yet.
Regardless, the Code of Conduct still applies and posters are subject to being reported and moderated.
That is correct. Always funny to see.
Hmm maybe something left over from the open beta. Weird!
So at some point only people who actually purchase the game will be on the forum like the D2R and D3 forums?
Yes, at some point they will lock it back down again and only people with the game license will be able to post. The exception to that is usually the Tech Support type forum so that people can get help with install and such.
All Blizzard forums, with one exception (Battlenet generic Forums) require a license, license and subscription, or free license and in-game progress milestones in order to post.
I seriously doubt they would change that policy for D4 long term. It would be a mess of spam and trolling.