You suck it up and move on.
> Should employers always be on-guard of employees leaving at-will? ... should employer be able to fire any employee at-will?
I live in Texas which is an at-will state. Of course we business owners are prepared and ideally so are the employees. Each state is different but, even if it's not an at-will state, you as a business owner need to prepare for replacing employees. Your comments imply that you think that the worse thing that could happen is that an employee decides to leave. That's crazy. What if you find out tomorrow that your employee has been posting source code on a P2P site? What happens if the employee has been found embezzling/stealing? Or what if said employee had a heart attack tomorrow? Or their spouse/child died and they needed an extended leave of absence? Those lost two are horrible situations, yes, but they are very real possibilities. You need to have training/policies/documentation in place to help with such. We spend no less than 2 hrs per month per person on maintaining/creating documentation.
See my other post in this thread for more details.