Can they not purchase health insuance without the subsidy?
Is it possible? Yes, they're totally allowed to. Are they capable? No, they can't afford it.
If they could afford it, they wouldn't be subsidy eligible in the first place, a key part of this law.
The legal question in here is "do they have a RIGHT to access to this citizen-only subsidy as non-citizens?" The answer will be no, and this case will be thrown out. It's clearly defined in law, and the law will be upheld as written.