Why do some parents think they are allowed to take their stress out on their own kids when they are not?

When the kids create the stress they need to be taught that there are replication for their acts.

cause theyre stressed and dont care

I was a punching bag for my mom sometimes

they are bad parents with mental issues. 

I know some are under the delusion that their kids will learn about stresses as an adult and their effect on their lives, some maybe are so concerned with their safety that they end up being blind to how much they are hurting them, and some are just bad parents.

The best thing you can do is to not do it, and when you slip up (as we all do), make it clear to them after calming down that your behavior was unacceptable and that you choose to react poorly to the situation so the blame is entirely on you and not them.