It's more like paying for a movie soundtrack or art book before the movie is released.

IMO it's not abusive, just offering people options that they can completely ignore without affecting gameplay at all.

If they were working on selling an expansion when the base game isn't done, that's different. Though even there, I'd say it's more an issue of priorities and ultimately would come down to how much do you have to pay for each piece of content and are they worth that price. And if the developer follow through on delivering everything promised (or if they do a good job of tempering expectations before the sale).

And personally, I have more of an issue with MTX than DLC (and am not one of the ones who rail against DLC culture). DLC is offering more content for a price, MTX is using money to unlock content that's already there. Sometimes one can be more like the other (like DLC to unlock a costume is pretty much MTX that you purchase outside of the game and MTX that enables new levels are pretty much DLC you purchase from within the game), but it becomes really problematic when the game is designed to just incentivise you to spend more money.