well, I am a Shia and although I myself don’t follow these things and don’t agree with them but people are not forced to do this. It’s their choice. Moreover, Muharram is a period of mourning and flagellation is just a method (although it has not been prescribed anywhere) to understand the pain the family of the Prophet went through when they were brutally murdered.
These are probably their way of expressing their devotion and we should respect it even if we don’t agree.