Responses
While I too think it's a plot device to lengthen out the story
I think the only plausible reason Jinha isn't telling Siwon is that
>tells Siwon
>Siwon understands and stays away from baddie
>baddie boy is like damn you know now, now I can't have my fun
>baddie does crazy rich guy bad stuff and possibly hurts Siwon (which we know Jinha wants to avoid at all costs)
Maybe he's overprotective to that extent, that he doesn't even want to tell Siwon bad things. Like a parent refusing to tell their kid about the harsh reality of the world.
Yours is plausible as well.
By this point it just makes no sense. Kang Jinha only has to explain to Siwon what is going on. There is absolutely no reason not to. This is just a lazy plot device to lengthen the plot.