Consider "obscene" speech/media/etc. Consider hate speech. Or look up "google vividown lawsuit" (the defendants all live in the U.S., and no one cared about that fact). And let's not even get into national firewalls (e.g. China, half the Middle East, etc).
With respect to the phones... first of all, it sounds like we agree about the likely odds of individual phone users being taken to court. There's no way that anyone would be individually prosecuted for this, without there being an ulterior motive. Of course, that's a big "without"... the RIAA/MPAA seem to be utterly free of compunction, and there are so very many cases of prosecutors bullying people with obscure drug laws for various incomprehensible reasons. "3 felonies a day" and all that.
Second, yeah, if you run a business in France unlocking phones, and some of the phones you unlock happen to belong to U.S. citizens vacationing, it's not clear that the US would claim jurisdiction over you. IANAL!!! But if you run a business in NYC unlocking phones, yeah, you've got a problem.
Did I mention that I am not a lawyer, and this is not legal advice? 'Cos I'm not.
The UK was allowing him to be extradited?? I don't feel so bad about UK libel law applying to US companies anymore (although it hurts everyone).
Also, O'Dwyer had his domain name seized from him by the US somehow.
I doubt most US courts are likely to say, "well we don't have jurisdiction over __that__".
Although, such cases that come to my mind were websites, which operate internationally, even if hosted entirely outside of the US. This is obviously a different situation than the seemingly commonly-cited "underage drinking" examples. For the people who build unlocks, though, like MuscleNerd, that is apropos, and those are the people who I mostly spend my time contemplating (although whether the DMCA exemptions actually help them much is another question).
(For individual users messing with their phones, of course, most of this is pointless: there is really no practical chance in hell that individual customers will be taken to court over things like this; it would be more a looming situation for people who offer this as a service at their store, which is quite common at phone repair shops.)