Yes, it's true that it's hypocritical that people get worked up about animal sacrifice in general but not about factory farming, millions of turkeys being killed for Christmas or Thanksgiving, cruelty in slaughterhouses, and so on. Most sacrifices are probably carried out fairly humanely, with the minimum of suffering to the animal.
But the animal welfare campaigners are campaigning on many issues (including factory farming), not just the mass sacrifice.
And Gadhimai Mela is a mass sacrifice that is not carried out humanely.
According to the Humane Society, the animals are not killed humanely - hence the reason for the protest:
"Cruelly, the animals are chased and hacked to death with knives in a competition to kill as many as possible within two days."Here's an account by a Nepalese eye-witness of the event (takes a while to load, but should be read):
The sword-bearers cannot chop off the buffaloes' heads at one go because of the thick size of its necks. To make their task easier, the hackers first cut the buffaloes’ hind legs after which the animal falls on the ground. They then start hacking the neck until the head is separated from the body. It takes 20 to 25 swing of the sword to annihilate a big buffalo. The suffering the animals go through is unimaginable.This is not on the same scale as the sacrifice of the occasional chicken in Santeria, where the chicken gets eaten.
After witnessing the Gadhimai carnage, I started having terrible nightmares. I would see blood wherever I turned to look.
Three to four days after the massacre, people start fleeing the Gadhimai venue because of the nauseating smell that starts to emit. Cars, rickshaws and cyclist start taking alternative routes. It is the people living in nearby localities who suffer the most. While the temple area turns into a breeding ground for disease, many fall sick. It takes months for the smell to go away.That does not imply to me that all the meat from the slaughter at Gadhimai Mela gets eaten.