I think they just have more faith in the theoretical science.
There's a lot of stuff coming out about viral load and how asymptomatic or mildly infected people are giving off very low doses of virus. So if you catch it off them, you should also get a very mild version of the disease.
Sweden are telling the vulnerable and ill to stay home, and they've thinned out mass gatherings, so what they're aiming for is that at a controlled rate, very mild or asymptomatic people spread the disease at very low doses and the healthy young people they are infecting will get it mildly.
If the spread gets too quick, or the healthcare system starts to get saturated, they will go into a full lockdown like everyone else has.
The thing with Sweden is they have the benefit of very low population density, even in their cities. Stockholm has 1/3 the population density of London, and that's the biggest factor in how fast the spread is, so they are in a position to try the controlled exposure theory that no one else really is.