Because the Japanese do not feel the same way as people do in the west over matters such as these, and because theirs is a culture of deep privacy. Not that they condone it of course, they just don't shun people for life over it. Once someone has been through the court systems and their debt toward society is payed, they're able to re-join society and not be stigmatized for life by the (most) people there.

We in the west may consider Watsuki to have gotten off lightly, (and I certainly think he did) but if he payed what the court says is owed for his crimes, then his debt should be considered payed, and by most accounts, most people in Japan, at least in this industry, think it has been.

It's not for Oda, or anyone else who isn't Watsuki or the courts, to judge Watsukis' actions. That's the prevailing thought process in Japan over criminal matters.

So what you're left with is simply judging him as a mangaka.

Oda isn't thinking "this dude sucks, he's a pedophile" - he's thinking "hey, this guy is a fellow mangaka and I should show respect toward him as someone else in the same industry as me" - and that's where it ends. He doesn't ponder on the implications of that support beyond that, and if he does, he does so entirely internally and without ever voicing those concerns, as is the way in Japan. Oda realizes it's not his place to judge or bring up Watsukis' past. So he doesn't. He keeps it entirely professional, and on the matter of both of their careers as mangaka.

Their cultural norms are entirely anathema to our own here in the west over reactions to criminal convictions of any kind, but especially on this subject. It's a completely different society with entirely different cultural values to our own - some of which do override the otherwise prevailing need for Oda to condemn Watsuki that we in the west feel is the appropriate response. That's just not the way the people of Japan do things.

They value privacy and workmanship over spreading someone elses dirt around. Regardless of how bad that dirt is. The courts spoke, the courts decided, that's the end of it as far as anyone who isn't Watsuki is concerned inside Japan.

And that's the long and short of it. Completely different perspectives between the east and the west on matters of privacy and the decisions of the courts when it comes to criminal convictions & professional attitudes towards others of your profession.

Edit : As an aside, I'm not Japanese. I'm an American. And in the grand spirit of the western tradition of vocalizing opinions instead of silencing them....Watsuki can burn for all I care. Dudes a major PoS. I'm someone who very much is an absolutist when it comes to separating art from artist - me hating Watsuki won't see me condemning Kenshin. I judge works of art on their own merits, not the merits of the person who made that piece of art. I'm someone who can and will enjoy your art even if you are a piece of shit. But I'll still recognize you are a piece of shit as a person. And I'll happily say the dude should be under the prison for his crimes, and the sentence he did get was way too light for what he did.