For some people, this would be totally extreme, but for local Thais, this is completely normal. They ignore the pain because this ritual is a rite of passage to adulthood. This is an annual ritual performed in public to announce to the whole world that they have become adults.
The Kayan women of Northern Thailand are known for the exquisite and incredible brass coils they wear around their necks. They are often called the "long necks" or the "giraffe women" by outsiders and can wear up to 25 coils, which many never take off.