>At what point do they become subject to Illinois law?

When they sell to someone in Illinois, although there is probably some bullshit case law out there that says the website owner has to take basic steps to block Illinois users to avoid all IL jurisdiction.

>And can Illinois prosecute someone for doing something that is legal in California while they are in California?

No.

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bluGill 2 days ago | parent [-]

But CA cannot stop IL from demanding someone be sent to IL courts if they think an IL law was broken.

	▲	lotsofpulp a day ago \| parent [-]
		IL laws can't be broken by someone not in IL that does not interact with anyone in IL. Presumably, there is a defense available in court that would prevent CA from extraditing someone to IL who is innocent because they have nothing to do with IL.