It's not paywall behavior it's just a different race DLC like they've done since at least Rome Total War when they sold Allexander as an expansion. Shogun 2 had many races as DLC as well as Rome 2. This is the way CA does business, it really shouldn't be surprising at all.
This has been the DLC model for Total War games since Empire. I knew with the massive amount of factions in Warhammer that this would be the case. Definitely wait for a Gold/Complete edition if you don't want to get hammered on DLC. There will be a TON for this game guaranteed.
Absolutely there will be DLC races. I doubt anyone could realistically expect them to make every race in the WFB universe for launch. We know that. As a few other people have said, however, is that
it's Chaos. Chaos is far and away the most popular army in WFB, is at the core of the majority of the story that happens in the lore of that game, and will almost certainly feature as a major antagonist in the campaign(s) that you do get on launch. All of this means two things:
First, there was a deliberate decision to take the one army that damn near everyone is going to want and decide from the outset that they won't include it unless you pay extra money, meaning that for many people the game will not feel "complete" without at least one piece of effectively-mandatory DLC.
Second, the first piece of DLC will almost certainly consist mostly of assets that have been made for the base game. I'm assuming that Chaos will feature in the campaign of the main game, so most of the animations, artwork, unit rules, behaviours, etc. will already be in the game, you just can't access them as a playable race unless you pay a bit more money.
If they did this with basically any other race except Chaos or the Empire, I would be fairly content to say "well, of course other armies will be DLC. Sucks that it's a preorder bonus, that's always scummy, but I can cope". Putting one of the two most iconic races in the setting, and the experience that they know the most people will want, behind a paywall AND using it to incentivise preorders of a game whose immediate predecessor (Rome 2) was a buggy mess when it launched is egregious, cold and very clearly calculated.
Honestly, I actually don't care all that much. It just means that they won't get my money for this game until the "complete edition" is $30 on Steam in a couple of years. They could have got $60 out of me in launch week, easily, if they hadn't pulled quite this much crap. Oh well...