In theory it seems to be against the rules. In practice it isn't.

What is the reason for not punishing these people?
Are there too many of them to deal with?

I have been active here only for about 3 months. Could someone please explain why trashtalk is allowed?

And please, don't recommend blocking the user, most of you probably know that it isn't a solution to the problem.
Blocking prevents future abuse from happening via PMs.
We are talking about an abusive message sent before adding the block.
Not to mention that blocking the abuser doesn't prevent him from abusing other users.
It doesn't require a lot of thinking to figure these out.

BTW, the trashtalker I met has an account registered in 2007 - it has a star and more than 10k forum gold. If the account isn't second hand then we are talking about an adult being with a primitive mind, these people rarely improve so this was probably not his first abusive PM here.
AFAIK the star means that he purchased forum gold with real money - that can be basis for a conflict of interest when it comes to actioning certain accounts here.