Did they take them there because it thwarted their ability? Or did the characters happen to do things that would get them taken to Strahd?
Two of CoS's hooks are "Someone asks the PCs for help. If they agree, the Mists come and they get whisked off to Barovia."
One of the hooks is "The PCs hear about werewolf attacks and go to investigate. Then the Mists come and they get whisked off to Barovia."
The last hook is "The PCs are walking down the road. Then the Mists come and they get whisked off to Barovia."
When I ran the adventure, I did what I usually do: have the PCs be natives to Ravenloft, because I hate the "weekend in hell" concept.
Anyway, the actual point is that some people were saying that it's not their job as a DM to tell the players that their choice was going to be nerfed (I would consider that to be a common courtesy), and you'll note that in none of these hooks are the players actually informed that they're going to another plane of existence that they can't willingly leave.
(Do most people usually choose backgrounds because they will explicitly be useful, or because they sound like they will make an interesting character?)
I think most people choose their backgrounds with the idea that their background feature won't suddenly become worthless because of DM fiat.
If someone is a fire mage, what effort should the DM do if there are some places on the map that currently have fire giants or a red dragon?
Bad analogy. It's more like if the PC is a pyromancer and the DM deliberately gives every single enemy resistance or immunity to fire damage just because. Bandits, ogres, frost giants, white dragons--all resistant or immune to fire. Oh, and they didn't bother to tell the player ahead of time. Why? Because the DM has decided that it makes sense.
See, the difference is this: If I'm playing a pyromancer, I do so knowing that sometimes my chosen damage type is not going to be useful, or it may even be a serious hindrance. But I also expect that sometimes it will be useful and sometimes will be super effective. That's exactly what the game should do. I
shouldn't have to worry that the DM may arbitrarily decide to shut my ability down.