From the article:
Police in the canton of Schaffhausen, in northern Switzerland, confirmed the arrests, while the public prosecutors’ office confirmed it had opened an investigation into suspected incitement and aiding and abetting of suicide.
The person who died was reportedly a 64-year-old American woman. Switzerland is one of the few countries in the world where assisted suicide is legal, under certain conditions.
But the article does state that the interior minister does question the morality and legality of the device:
Switzerland’s interior minister, Élisabeth Baume-Schneider, questioned the moral and legal status of the Sarco Pod, a device that is designed to allow a person inside to push a button that injects nitrogen gas into the sealed chamber.
It’s hard to say why the arrests happened without more details, but I’d suspect the nationality of the individual may play a role.
I would agree on the investigation ensuring everything was done diligently and to protocol. I don’t think it’s some international political issue that the US is waging on it, just that they are an individual traveling to be euthanized it might be seen as murder in the individuals native courts. The United States still having issues letting people go state to state for treatment, let alone internationally, causes me pause in such situations.
I’ll be the first to admit I have no real knowledge on the laws that apply here but the United States has been known to inject themselves into other nations matters regularly. This is again just opinion and no way substantiated by anything tangible.
Edit: To add to this, this MSN article seems to give additional information. Specifically, the following quotes:
So most likely this is due to being the first incident of voluntary euthanasia being legally done in Switzerland, and ensuring the legality of such procedures via precedent for future such cases.