Who's "they" and why are we paying to uphold their power to make this decision whenever they want
You probably don't pay them to uphold their power to close roads whenever they want, it's just a consequence of the aforesaid monopoly on violence.
If you think it's wrong, you can try to bring competition into the market, but if history is any indication you'd better make sure you have a lot of upper/middle-class people who agree with you first.
A parade or block party closure is a closure of a public road, so is closing the street and digging a tunnel.