To those who remember the horse meat scandal in 2013 in Europe, a recent donkey meat scandal in Pakistan may not appear particularly surprising. Food adulterations are so prevalent that people may laugh rather than worry.
This is probably not because people do not care about what they eat. It is rather because there are few things they can do to avoid mislabelled foods. I believe, that laughing is a psychological defence scheme rather than a response to something really funny.