It is an extension of the UAE anti-drug policy. They see that prescribing opioids for pain management is going to lead to prescription drug abuse, particularly when it comes to Tramadol and its different versions. The same thing applies to tranquilizers and other medications that some people use recreationally. Your affliction or disease must fall really high on the pain scale before a doctor of any specialty - let alone a GP or dentist - will prescribe something strong, and each such prescription is closely monitored and registered with your Emirates ID, and each doctor's record of such prescriptions is regularly reviewed by the health authority of the Emirate in question, and you can't get two different prescriptions from two different healthcare providers unless it has been establish that 1) your first prescription has been exhausted in line with the specified dosage, and 2) that your pain score - by UAE standards - still warrants the painkiller.
Some of these are sound and successful policies, but it does get ridiculous when the pain score is determined based on diagnosis without factoring in the patient's physiological ability to withstand pain. In other words, you'd have to be decapitated before someone will give you a shot of morphine.