It's simple: if you do something which causes something illegal-in-the-US to happen on computers-located-in-the-US, you can be prosecuted by the US for it. There is nothing new, radical, revolutionary, terrifying, unprecedented, etc. about this idea. There wasn't anything new about it recently when some bitcoin people were busted because they laundered their proceeds through US institutions. There isn't anything new about it when someone in another country hacks a US company's systems.
"The internet" is not a magical extra-territorial location which wipes away all concept of countries having jurisdiction. As long as a computer in one country can cause things to happen to/on a computer in another country, there's an opportunity for either country, or both, or others in between them on the network, to have jurisdiction over things that go wrong.
And your "only a matter of time" slippery slope has already happened: several EU nations have taken legal action against US-based online retail/auction sites to require compliance with laws against selling Nazi symbols or memorabilia. And I hope you'd understand that no matter how much you might argue "but I wasn't even in Germany", Germany will come after you if you offer to sell and ship items to Germany which are illegal to sell/ship there.