The problem is that it is actually rare for you fine ATCOs to refuse any request to avoid weather and some drivers get into the nasty habit of turning without requesting. That is done purely on the assumption that they always get what they want when avoiding weather so why ask in the first place. It is a classic example of complacency and it is a poor show really. A short sharp word on the R/T might work if the frequency is quiet but failing that then report it.
