Paulenrique already explained how things work very well, but I feel the need to point out that this quirk in Portuguese can be used for literary effect. For example, there's a very famous story about a famous Brazilian writer (Eduardo Portella) who was once an education minister; he was a left-wing person working for a right-wing dictatorship and, sure of the temporary nature of his position, started his first press conference with “Eu não sou ministro; eu estou ministro”. This quote became famous for its elegance (and, surely enough, he was out of the job a year later).
wow..it's pretty interesting to teach your own language to others because we face things we've never thought about! Yes,your state of being a tourist is temporary, but that's not something that you "get" (Estou com sede / cansado / esgotado). It's something that is on you, so to speak... well, got it? (I hope so :-))