I see this a lot and I find it disturbing and dangerous to both the cham and the keeper.
Chameleons naturally climb up to escape danger. They feel threatened when cornered. Stress degrades your chameleons health.
When a chameleon defecates it rubs it's vent on the branch or vine this in turn contaminates the branches with intestinal parasite eggs or parasite cysts. The chameleon then walks on the branch contaminating its feet.
Letting a chameleon climb on your head and face is a sure way to expose yourself to many species of parasites.