Cloverjo, its not as simple as that (is it ever??) Its to protect staff from allegations of drug theft/misuse. Additionally, it puts staff and their cars at risk of people believe that they may be carrying drugs. It used to be that if NHS staff took drugs back to a pharmacy for disposal, the NHS organisation they worked for got heftily back charged by the pharmacy for the service. If a member of the public does it, its a free service.
When I worked in the NHS, I used to visit people at home and quite often found large amounts of unused meds. At the time, we were allowed to remove them for disposal if the patient (or other house resident) was at a risk from the situation but otherwise, we were allowed to reorganise the meds, so only those in use were to hand, and put the others in a safe place but we had to ask a friend or reli to take them to the pharmacy. I can’t remember what the charge was now but it was charged per item and fairly hefty.