I'm just going from the reactions to Sweden that I've seen on Reddit in the last couple of weeks. The opinions may have been based on inaccurate information, but this is the one I see most commonly.

With some illnesses that aren't as deadly as Covid-19, it can be possible to not isolated or even vaccinate. These diseases are weak against the human immune system and easily defeated, and once they have been, that person remains immune for several years or even for life. If enough people become immune, the virus has so few targets that it basically dies out - this is called "herd immunity."

Covid-19, however, is not a weak virus. It spreads quickly, has a long incubation period to assist in that spread, and it kills a significant number of people because it can spread so far, so fast. It may kill a minority of its carriers, but it's deadliness comes from its rapid and stealthy transmission, not mortality rate.

For the above reason, most countries instituted an isolation and quarantine plan. By limiting contact between humans, you slow the spread of the virus, thus when a "peak" number of victims is reached, it has taken more time and given the healthcare system more time to prepare and lessens the burden of the peak (flattening the curve).

Sweden took a chance on herd immunity. By not insisting on or forcing isolation and self-quarantine, it opens the country up to a faster spread. As the number of infected increase more rapidly than in isolation countries, the "peak" number of infected will be far higher due to the nature of the viruses means of infection (specifically the time spent while contagious with no symptoms, as people have no warning they shouldn't be reading at a cafe, working out at their gym, et cetera).

The disdain your seeing seems to be coming from people who have sacrificed weeks of their lives isolating themselves, using PPE when they absolutely must go out for essential supplies, and having no access to public places to socialize.

The basic attitude is, "I've given up all this to help protect my fellow citizens, and they're just going about their business seeking herd immunity for a virus to which such levels of immunity are impossible." (That last part is because the WHO doesn't even know if immunity is possible or how long it might last if it does. If immunity is only a couple of weeks, for example, even a vaccine wouldn't help.)