Lots of people don't clear samples all the time. It's stupid when you're famous like Timbaland though because then people will want to come after you because you're making money. It's a waste of time to go after many small-time producers who don't clear samples because you're not going to get anything out of it.
That being said, I give Timbaland a pass because Nelly Furtado is super hot and has an awesome accent.
We live in a remix-culture in all senses of the word, from fashion to music, to everything. Things get blurred, it happens. I'm not super worried about that always, IMO. It's part of the creative process, almost everything we create has referents and influences.