He made himself subject to the jurisdiction of the US when he decided to try and decrypt US classified information. This is also why the Russians were detained. They made themselves subject to US law when they decided to get involved in US elections.

If I conspired, from California, with people in Tibet to circumvent Chinese law, I also would not expect to be detained by U.S. police.

That is because the US does not have an extradition treaty with China, so it doesn't matter what Chinese crimes you are accused of committing. Though generally, the US would not extradite for political crimes, so if your Tibetan activities involved organizing political resistant or independence movements, you would be protected by the First Amendment while in the US.