Yes, it is.
I don't know where this rumor started that once a job has been cut the employer is prohibited from changing their mind or discovering that they've cut too deeply and are stuck with the cut, but no such law exists. Neither is is illegal to cite a non-existant "reduction in force" as a reason to fire someone (some employers are cowards, what can I say). But unless the employee whose job was eliminated has a valid and supportable reason to believe that the job eliminiation was a pretext for an ILLEGAL reason (a reason that is specifically prohibited by law - race, religion, national origin, applying for FMLA, filing a workers comp claim, etc.) it falls into the 'possibly unfair but not illegal" category.