Tbh, I kind of think it should be. Not de facto illegal, like if you accidentally burn it somehow, but if you intentionally do it to piss people off then that intention isn't exactly right itself. If you're putting on a public display purely to incite and antagonise people by destroying things they hold dear, then you're not merely exercising your freedoms but actively seeking to harm others.
It's all very grey area though, and any punishment should reflect that the harm is not physical and relatively low. This law almost definitely goes too far.