Oh they burnt more than just dumpsters. And you're right, the police should have handled it, but they didn't because the city didn't want them involved in simple property damage, regardless of the severity. They were apparently concerned about how the media would portray their efforts.
As a former cop, or even as just an old retired guy, I would never encourage vigilantes but I do stand with anyone defending their life or businesses from the "mostly peaceful protestors" who ran amuck in Kenosha, causing damages such as these, somebody has to.