My assumption is that in the gaming industry, between development and publishing, there's some steps in between where the game is done, tested, but "STUFF" is involved in order to get it out to market.
So it's like someone's developed a program and has fully tested it, and then decided to put that out for release.
And while it's being reviewed by other teams and whatnot, the team decided to add more stuff (extra music, extra art, whatever).
Once the game hits the market, we now have all this extra stuff that was unavailable in the version that was sent for publishing. What to do? $$$ it.
Under this assumption, I don't really see an issue with day one DLC. There are devs that will likely set some timelines in advance. They may have announced it on their blogs or websites, or put it on their crowdfunding campaign, and now they're kind of stuck with it. They can of course move the dates because they're not obligated to follow it, but that's not a good idea.
And for whatever reason, if they're behind schedules or things happen and they're unable to have all of the content implemented before the deadline, they may have a fully tested version available without all of the content (eg: extra quests, maps, objects, and so on).
I do not see any reason why the three points you have should be a day one DLC. Just have it ready immediately, or wait a week. No harm done.
But yes, why does it have to be released on day one?
Just release it later on. Advertise it as a separate product. Use it as a way to prolong the life of the product. I don't see the point of day one DLC.
I have a few manga, wallpapers and soundtrack to give in Deluxe Edition. I fail to see negative side of this practice as it is up to buyer if he/she believes it is worth the extra price.
Which is fine. It's like when Blizzard was selling those limited edition packs/boxes for premium prices. Players of course had the option to not buy them and just go with something cheap. I'm sure there were people that felt entitled to the premium box set, but they probably didn't get much support for their rants.
But I'd say this is different from day one DLC, especially if the content was already on the disk or whatever media you used to deliver the "original" game.