On the Manage Pollers screen, why is Edit grayed out?

I'm hoping I'm just being dumb here and missing something obvious, but when I go to the Manage Pollers page and select a poller, the Edit option is grayed out. This is the case no matter which poller I pick, and whether the poller has nodes assigned to it or not.

thwack-cant-edit-poller.png

Any ideas?