Example?
Because these people are emotional, not automatons. People get happy, sad, pissed off, annoyed, angry, frustrated, ecstatic, etc. Especially on the field play at a reasonably high level. It's incredible that so many people expect cricketers to be so passionate about the game that they're told they shouldn't make a career choice by playing IPL and should only play for their country yet we should also ask cricketers to not show passion when it manifests itself negatively? Sure, go nuts when you take a wicket or score a hundred but keep your trap shut if something bad happens to you? Come on.
Take the good with the bad; cricket isn't just a metaphor for life for these people, it is their lives. Everyone gets angry/frustrated/etc.in real life but we're afforded the grace of being able to express it in private. Sure you could argue that cricketers should only do so in private too but in the heat of the moment, stuff happens, tensions are inflamed, stuff gets said. It's a highly competitive environment with quite vicious performance pressure and people snap. As Goughy said too, being a quick bowler is bloody painful and if some bloke gets a plumb LBW go his way after you've spent half-an-hour setting him up for the off-cutter, you're going to be upset.
Anyway, forgetting the ethical consideration, it's psychologically unhealthy to bottle stuff up and causes more violent reactions when your emotional bucket is overflows. Banning players from saying anything rids the environment of a tension relief valve; if players can say the odd word to each other, the chance of a huge blow-up is lessened. You watch; if sledging were to be outright banned, the tension on-field would increase markedly and we'll see a physically violent confrontation. We already see the same in real life.