The range 1:50 is only used in the plot, not in the rest of the example. See Documentation on FFT I guess they chose 50 because they want to demonstrate that it is very difficult to see frequency information from a noisy signal, that's all.
When they get back the information they want, the frequencies 50 and 120, you have to use the full signal, as they did. They actually pad to the next "power of two"-size, to get a more efficient transform.
