Mental health units generally take a very hard line where drugs are concerned because many drugs, when combined with psychiatric medications can cause major problems and even kill. Inpatients are often very vulnerable and the effects of other patients upon their mental health can be devastating. Clients admitted to psychiatric hospitals who set about exploiting, ridiculing or otherwise distressing their fellows are generally 'moved on' very quickly by the ward team. It's not a good way to get help for yourself and it can be very damaging to the care of your victim. This is also why those patients who demand a lot of staff time and attention will only get it if the staff think it's because of genuine need.
Time spent with one patient is also time taken away from another.
Many people are surprised to learn that this is also considered an abuse as it prevents other patients from getting the care they need. Bear in mind that this does not mean that inpatients are expected to sit quietly and wait for their medication like good little girls and boys. Mentally ill or not adults are adults and have a right to express their needs, fears, distress or whatever. They're also entitled to friendly conversation and many clients do strike up friendships with professionals as a result. It's simply that attempts to monopolize staff time for non-genuine reasons cause problems for patients and staff alike.