Doing things that are costly isn't the only way to reliably signal. In this case the rabbit reliably communicates awareness of the fox's presence. It cannot be fake because the rabbit must look in the right direction. The fact that it's prey is not unaware of its presence is always going to be useful to a fox. It will still attempt to chase aware rabbits some times but the exchange of information will help both creatures in their decision making.

This is an equilibrium that all else being equal will be stable. Speed will still be selected for for exactly the same reason that it always was.