This is an interesting read
China.com.cn made an article about the wave of tumblr users signing up to tumblr. And I have also realized why lofter has more focus on creative commons/copyright licences thanks to Ms. Roroll.
About Tumblr: We can find many types on tumblr like art, novel, photography, fuckyeahxxx and so on.It’s very free.
About Lofter: You can only post what you created by yourself, you should assure your post is original.It’s a website for artist.
So the licenses are to cater to the users who create their own content.
In that case, reposting content which isn't yours (aka pixiv images) is prohibited here. For the record, I've been here since February and only post text and my own stuff here but I think it's really important to realize what you're maintaining on your own blogs.