If pot/marijuana is illegal in the US, then why are so many people not getting caught?

So my question is why are so many people in the US (ie. stars, high school kids you hear talking about smoking pot) not getting caught? For example a star that was in the news: Michael Phelps. He was in that picture, I think, smoking weed from a bong. And Kristen Stewart, the person who plays Bella, also smokes weed/pot. I think weed and pot are both the same? Is there an explanation for this? I'm not blaming the police or anything; I think I just got it mixed up.