Revealing mistake: When Johnny shows up to Cobra Kai dojo and all the kids are there. In the video playback they show on the phone you can see Miguel didn't land the kick, and the kid just fell down. (00:28:18)
December 19 - S3-E10
Revealing mistake: While Kreese is fighting Johnny, the stunt double is clear in several shots.
Revealing mistake: When Sam and Tory confront each other at school, the same extra (a girl wearing glasses) appears in two different places. She is behind Sam during the confrontation but Robby also runs past her in another shot.
Revealing mistake: When Silver cuts open the box of new equipment, the razor knife has no blade. (00:05:53)
Answer: Just like in the original film, some guys want that brutality. Johnny in part opens the dojo because of his past rivalry with Daniel and to train a kid that's being bullied. But also, other kids that are being bullied want to join in order to learn to fight back. Johnny is also teaching the Cobra Kai creed (Strike First. Strike hard. No Mercy.") so that's why he opens his dojo with that name and the old Cobra Kai dojo's past reputation is exactly what he wants to elicit.
Bishop73