Bettas have been selectively bred for heightened aggression; wild
splendens are no where near as aggressive as modern bred fighting plakats. And, in the wild, they have ample opportunity to simply display, exchange a blow or two, and part ways - whereas in captivity, they are typically confined together in a small jar for a fight and thus can not escape even if they wanted to. There is nothing "natural" whatsoever about betta fighting, and the thought that it is somehow acceptable just because wild fish fight for territory is about as idiotic as suggesting that dog fighting is OK since wild wolves fight for territory.
Fish sustain fin and body damage in fights, which inflicts unnecessary pain, stress, and potential for infection. While the fish are not generally allowed to fight to the death, some do die from stress, injury, and infection after the fact. Even if most live, how does that make things better? Animals mauling one another for amusement is inhumane and unecessarry, period. "Culture" and "tradition" is not a blanket excuse for everything; cultures evolve and the fact that some stupid, inhumane crap has happened for many years doesn't mean it's just fine and dandy to let it go on unchalleneged. Even Thailand - where this all started - has banned betta fighting for betting purposes, which has significantly reduced the fighting hobby.
ETA: For those who think they just lose a few nips of fin, here's a pic from a well known betta fighter's site:
(c) fighterbetta.com
Oh yeah, nothing cruel about that *rolls eyes*
And don't even get me STARTED on the stupid little SOBs in the US fighting their petstore VTs and CTs because they're trying to look hard. Nothing drives me up the wall worse than seeing this little 17 year old ****s on YouTube with their sickly crowntails duking it out in a dirty, unheated bowl. I've been doing betta rescue for five years and if I get in one more emaciated fish with infected fins and big hanks of scale and gill plate ripped off because someone gets off on watching animals hurt each other, I'm going to start taking heads.