Sadly these things do exist. I saw one in the Hamster section on ebay just the other day while browsing. They are gimmicks pure and simple and in my mind, just another way for a child to potentially harm or even kill a hamster.
These commercial people should be stopped from selling stupid things like this. Harnesses are for horses, not hamsters. I would like to meet the idiots who think these things are a good idea and ask them if they have ever owned a hamster because it`s pretty obvious the people who make and market these things are doing it to make money and appeal to the easily led out there.
Hamsters live in natural plains, deserts and sub-lands. Their environments should mirror this in captivity where possible. Anyone who thinks a hamster harness is a good idea should`nt own a hamster. Sad, but true.