I don't know why it's so fun, specially seeing as I have never tried it out. Though, I suppose it's the same as every other RP. - Some people might enjoy RP'ing this way, not because they are violent or because it releases stress, but because they know what they are doing/they have things in common with other people there.
I have often seen these in Ficho as well, seeing as I've stopped RP'ing for some odd reason, I choose not to approach. I believe they've been around for a good while as well.
Perhaps it's the whole "arena" deal which attract them there. Like old Rome and their Gladiators.. Who knows..