Perhaps because US government wanted to do it despite TikTok not breaking any serious provisions of law this law has been made.
It feels like a sleight of hand from government to ban something that has broke no (serious) law (yet).
Did the SCOTUS go into the necessity of having this law to achieve what government wanted, if existing laws would have sufficed, provided that government met the standards of evidence/proof that those laws demanded.
If not, it is as if government wanted a 'short-cut' to a TikTok ban and SCOTUS approved it, rather than asking government to go the long way to it.
What this line argued in the Supreme Court in the oral arguments or in the opinion or in the lower court?
Obviously TT could not have brought this up, but the court could have brought it up while examining the government.
Only the appeals court (and presumably the district court below them) heard arguments about it being a bill of attainder. The supreme court chose not to. With regards to being a bill of attainder the appeals court appears to be of the opinion that it is enough that it isn't a traditional punishment, and that the justification for it was something other than punishment, without analyzing whether the government had a legitimate interest in achieving their non-punishment purpose. Of course they had already found that they did have a legitimate interest because of the first amendment analysis, but I don't believe their opinion with regards to it not being a bill of attainder relied on that.
Was it established that existing provisions of law is not sufficient to deal with the issue(perhaps not so easily as by fiat as in the new law, but requiring stricter standards of trial and evidence), necessitating this new law?