They use these because they're objects it isn't inherently criminal to possess. That's why you see so many antifags carrying around skateboards but never riding them. Sort of like Clanton's bike lock, it's a seemingly innocuous object, but can be deadly used as such.
It's still considered assault with a deadly weapon if you actually get charged with assaulting someone with it, and they can also throw in a weapons possession charge in some jurisdictions if the purpose of possessing it was for use as a weapon, which is usually fairly easily proven by the fact you did in fact use it as such illegally.