Most US states are at-will states, meaning they're already immune from wrongful termination, except for discriminatory reasons. No matter how long the employee has been there.
While "At-Will" in theory means that they're immune from wrongful termination, there's some major exceptions in most of those states.
See "Covenant of good faith and fair dealing" (which in some states mean you have to be fired for 'just cause') and "Implied contract exceptions".
I say these not to indicate you're wrong, mostly to make sure the entrepreneurs around here understand that even in a lot of "At-Will" states they might not be able fire completely with impunity.
w/r/t the GP as long as the rules around a trial period are set via an employee handbook or in the contract, generally in the US we're fairly free to try people out and let them go, the US is actually more liberal in this then a lot of European countries.
