Theres quite a difference between a Swiss soldier fighting for an allied country vs joining a terrorist group.
But therein lies the rub, doesn't it? Does the SDF or YPG actually count as a terrorist group? Does that definition spread to all paramilitary groups? If a civilian owns a gun and fires on a hostile paramilitary, does that make him a terrorist? Or only once his buddies join in to help? Maybe you need a silly acronym to really qualify, I don't know.
There's a little snippet that's linked to from the Swiss dude article, and it mentions a man who was tried for doing the same thing;
joining up with the YPG and fighting alongside them. The trickiness comes from the UK supporting the YPG and having sent them resources in an official capacity. If you then condemn someone who aided them as having "assisted a terrorist organization", then you kinda shoot yourself in the foot.
I mean, there are definitely very real issues with having people join up with random paramilitaries. ISIS is a great example of that; I'm sure the foreign nationals who joined IS all felt really vindicated for "helping the good guys". Armed forces of that sort are by definition less controlled and have fewer checks and balances, and that's usually a bad mix with the kind of power that lethal weaponry provides.
But from a practical point of view... Are the national militaries really that much better? In theory they should be, because they're supposed to have the intel from an established and trained system, plus a very large and very public organization that is to be held accountable for any breaches in international agreements its armed forces commit. But the actual effectiveness of those organs and how much they apply to the battlefield are sometimes a little sketchy, and then there's the question of whether or not they're doing enough. After all, inaction isn't a war crime; so it's generally "safe" to hang back and not do anything while others get slaughtered, displaced, or tortured.
It's a non-trivial ethical dilemma, and that makes me want to poke it with sticks.