Oh it was 100% broadcast and you're right - anyone could listen in to the police radio back then. Up until relatively recently actually. I used to!
Im just curious to what the theory is? Do people think Brewer is somehow part of a plot. That the plotters knew Oswald was going to duck into the store. And told Brewer to lie about Oswald looking suspicious? Or did Oswald not duck into the store and Brewer made it up being part of a plot?
I'm not sure what the idea is regarding Brewer and Postal. Some Oswald defenders say Oswald was fleeing the scene, the TSBD, because he believed he was going to be accused of the assassination. He realized he was set up.
So if he was indeed in flight - for innocent reasons - then it makes sense that Brewer saw a frightened or worried or desperate (whatever one wants to call it) Oswald. Oswald is, in this theory, afraid and that's what Brewer saw.
But the Oswald defenders want to see conspiracies everywhere EVEN IF the facts support their "Oswald was in flight because he was framed" theory. If one truly believed Oswald was in flight for his life then Brewer's description fits into that theory. Why refute it? It helps you theory.
Some conspiracy advocates are reasonable and thoughtful; some are not. This is an example of the latter.