Of course the dispatcher can’t give an order.
What do you think the dispatcher was trying to convey to Zimmerman with that phrase?
"What you're doing is not necessary for our purposes."
Seriously? The dispatcher was following CYA training that's likely in its manuals for dealing with Neighborhood Watch. With that comment the dispatcher was making certain that the City of Sanford was separating itself from liability if Zimmerman were injured while following the suspect.
Do you think Sanford wanted to be responsible for Zimmerman's acts and injuries had it directed Zimmerman?
It doesn't mean the dispatcher wanted Zimmerman to quit following the suspect . . . only that the dispatcher didn't want to direct Zimmerman to do so.