Although you will notice that some shops in Siem Reap rent a motorbike, this is really illegal. Unlike in most other cities in Cambodia (Phnom Penh, Battambang, Sihanoukville, Gabriel), the law prohibits local businesses from renting motorbikes to tourists. The reason behind it, is a bit unclear. Some say it is because of dangerous traffic conditions and roads in Siem Reap.
However, when visiting Siem Reap, you will see many foreigners driving motorbikes. This gives the wrong idea that hiring a person is allowed after all. The reality is different. Usually these foreigners are foreigners living in Siem Reap and they bought the motorbike they drove. This is allowed by law, provided you have a current Cambodian driver license. (What most foreigners don’t have, but it’s a completely different story. :))
 

I forgot to mention that it is possible (and legal) to rent an e-bike or an electric bicycle in Siem Reap. Obviously these are not considered motorcycles and so you do not need a driver’s license for them. Look at the picture below. You will find a number of shops across town renting these little green bicycles.