In Inception when every one wakes up in the airplane, Saito finally makes the call so I can understand he knows Cobb successfully planted the idea in Robert's mind. But I don't understand why Robert doesn't know that someone else has planted the idea in his mind. Robert and Saito both go to three levels deep and get lost in limbo, so how does Robert not know that someone played with his mind?
The dream, from Robert's perspective, seems very chaotic - just like any other dream. What is more, people sometimes dream of things they see while awake - the people around him. What is more, as Napoleon Wilson said in his comment, after waking up, the idea of the dream fades away.
Another argument in favor of the acceptance is that he was flying some public plane, probably owned by a well-respected company. Hence the mere thought that someone would be able to slip him some drugs and then connect him to the machine for a theft is pretty hard to believe (sounds very paranoid).
What is more, the inception is some sort of myth in that universe (remember the sequence in the beginning, where people tell Saito he's nuts, but it comes as a shock for everyone when Cobb admits to have accomplished an inception once).