it's the opposite arguement, it's like christians who believe shit without questioning it.
Christians didn't pick up the argument that atheists are immoral from the bible. They believe it's so because atheists have no rulebook like they do. It's a cultural phenomenon rather than a religious one. (Okay, that part is debatable I guess.)
Regardless, the argument goes like this: if you're an atheist, you don't subscribe to the bible, which means you don't follow the Ten Commandments (or indeed anything else that's written in there). Thus, if you're an atheist, you're immoral and would not feel remorse if you killed someone.
Then your argument in contrast: if you're not a vegetarian, you have no qualms about eating meat. Thus, you have no objection to eating dog or cat or anything else that might seem inappropriate.
Both of these are the same
logical fallacy.
why would you blindly accept it, just cause it the norm?
Usually, yes. That's how culture works.