Breaking windows is not cheap.
Car or building. Insurance or not. Especially on buildings. Windows aren't typically a uniform thing on buildings. With buildings a decade or more in age it's expensive because the builder has likely updated their selection and discontinued the old stuff. In many cases the window has to be customized or else the building is remodeled to fit a new window. It also needs to be installed right to avoid leaks of water and air.
Here in the United States, window replacement starts around $300 and for a lot of people that's half a paycheck. The bigger and more unique the window, the bigger the cost. I just had to replace my cracked car windshield and it cost me $350 and my car insurance wouldn't cover anything because the total didn't reach $500.
That's right, property insurance has deductibles to be met before they help with anything. So if my windshield was $499, I would have had to pay that, because that's "cheap."
As far as I can tell, "we break windows because it's cheap" is more of them admitting they caculated how much legal trouble they can get into if caught. Law enforcement are not going to investigate if the individual damage is under a certain amount because that's "cheap". If caught later, the window breakers probably spend a night in jail, pay a fine, but unless you the victim with a broken window can prove that they (one or two of a crowd of masked rioters) broke your window, they get off free and you are paying for it.
Even if it were actually cheap, people have every right to be pissed for the destruction of their property. Breaking windows is violent and has historically been a form of intimidation, frequently used for death threats.
People breaking windows shouldn't act surprised that when they are violent, people are violent back. Don't act like your violent, threatening actions are innocent and harmless. You're targeting people who very likely have nothing to do with a government's decision and who are probably against that government's decision. You're inviting even worse crimes like theft and arson.