Because regardless of whether they committed actual crimes or not, they joined a group that did this:
Sweden debates fate of returned foreign fighters
None of the 150 Swedish citizens who have returned home after joining the terror group Islamic State have been convicted of crimes committed while abroad. While some European countries are investigating all returning foreign fighters, Sweden is not.
But the country’s leading terrorism expert, Magnus Ranstorp, said investigations are the only way to “bring these people to justice.”
In an interview with news agency TT, Ranstorp, who is the head of research on terrorism and countering violent extremism at the Centre for Asymmetric Threat Studies at the Swedish Defence University, said the idea of investigating all returning fighters should at least be discussed.
“There is very little opportunity to bring these people to justice unless there are preliminary investigations of them. And it is necessary if you are to have access to go through, for example, their phones and social media accounts,” he said.