This is an issue I also noticed, I wasn't sure if it was meant to be like that though :S.

They can actually get out by using the half blocks at the back of the spawn to get on the fence though

I never plummeted to my death every time I had to go to entrolla, before /town spawn was fixed, I had to do /spawn and jump over the fence to the hill adjacent to spawn, then jump down to the Entrolla road.

If I am assuming correctly,(I don't know how the server permissions are set up,) spectators have no permissions on the server, and to use the portals you have to have the permission creativegates.use:
With out that perm, you can't use portals