Problem is that what "doing illegal things" mean changes over time, and in different places. What someone assumes to be legal everywhere, could be illegal in most places, and vice-versa
Further, if something's illegal in a different jurisdiction than you are in, you aren't usually exposed to any liability, as any tourist to Amsterdam will tell you. And not having a payment record doesn't change the fact you did it and will undoubtably have left other evidence.
Once again, these are social problems that magic beans won't solve.
You may not be exposed to liability back in the US, but that doesn't mean your Visa/Mastercard will necessarily work if you're doing something illegal in the US: https://www.cbc.ca/news/canada/toronto/square-canada-1.53031...
These are again, very much, social and not technological problems that require cryptocurrency.
In Canada for instance, you can easily get a merchant account with Moneris (a Canadian payment processor) and take card transactions over Interac (the Canadian debit network). Yeah, Square's bound by their US banks, because they're a US company with an overwhelmingly US product being sold internationally.
Ironically the government here in Ontario will sell you marijuana by mail and they accept Visa and Mastercard! The rules must be very vague and inconsistent.
My mental model was like yours, and like friends in college who fantasized about road trips to another state or Canada to find a bar that would serve 19 year olds. Not all laws are about the location of the act. Countries can and occasionally do bar their citizens from doing things anywhere, as was the case of Cuban goods.
There's also some weird extradition stuff that's been floating around lately that I can't make heads or tails of. There's definitely some fearmongering going on around it but I don't know if that means I should be concerned or dismissive.
And then there's one little idea. This may be crazy, but if the citizens of your country generally agreed that something was dirtbag behavior, you going somewhere where there is no law against it still makes you a dirtbag. Just a law-fearing dirtbag.
Ex post facto requires maintenance and vigilance, like any piece of safety equipment. Don't just count on it always being there. We all have to work at it.
So because the root cause is a (probably unsolvable) societal problem, we should just forget about technical workarounds like Bitcoin?