Why is this entry signal showing green?

Solution 1:

The problem is that you've got an entry pre-signal giving access to a block without any exit pre-signals. In that situation, it acts as an ordinary block signal, and since the block it's protecting is unoccupied, it shows green. In order for your signal to work as you expect, you need to turn the two signals protecting the station into exit pre-signals.

(Note that pre-signals are nearly obsolete. Path-based signals can do almost everything they can, many things they can't, and usually permit a more compact layout.)

Solution 2:

The reason that specific signal is green is because entering trains have a valid path to continue. Notice the signal to the left, on the other track; that's an exit, I'm assuming. By having the X in the middle of your block, the entry pre-signal is picking up your exit signal as a valid path to entering the station.

If you're trying to make one way tracks, it's a good idea to have it on the other side of the station. Barring that, it should be entirely separate from your entry block.