I spoke about this to a friend who is a lawyer. He told me this might be to protect the carrier against bad customers who maliciously claim they didn't purchase

I spoke about this to a friend who is a lawyer. He told me this might be to protect the carrier against bad customers who maliciously claim they didn't purchase the service in the first place, pretending it had been done by a fraudster using their personal info. A scam that allows them free Internet for ~45days until the carrier decides to call on the 1st outstanding bill. Apparently, a still picture of the customer in the shop wasn't a strong proof of presence, so they decided to make a short video (or GIF) instead.