Because that site is for American's (or maybe all of North America can access) only. That means that the rest of the world either have to go trough a proxxy/tunnel his connection to watch from that site, or they could just download with one click. Which sounds easier? Also some people wanna archive so they can watch from the harddrive. Although i go for HorribleSubs myself since I'm to impatient when it comes to OP, and i only archive KF. But then again i don't really have to fear getting sued over anime. I'm guessing they only sue over that in the US.

EDIT: But if i had access without any haggle i would watch it from a legit site like onepieceofficial.com. I think it's a great move to stream it for free and it should be supported so that others will learn from this.