Think of it this way. You could view this as the population mean for the random variable. So that's just going to be a known, it's out there, it's in the universe. So the expected value of that is just going to be itself. If the population mean or the expected value of X is 5, this is like saying the expected value of 5. Well the expected value of 5 is going to be 5. Which is the same thing as the expected value of X. Hopefully that makes sense.