While it does not necessarily make it illegal, per se, it most certainly does raise an issue of whether or not it could be subject to suppression in a subsequent criminal prosecution proceedings.
It would make an interesting suppression motion; however, the Army Reg does not affect any constitutional standard relating to reasonable searches and seizures or privacy rights, so from a strictly constitutional standpoint, which is the standard against which suppression motions are measured, it would not help you. Concealed video surveillance cameras are commonly used in places generally open to public or business customer use without posted notice and there is no constitutional violation. On the other hand, some types of surveillance has been made illegal by statute (for instance, audio recordings in Maryland are illegal without notice to the person being recorded), which would be a basis to bar the use of such evidence at trial under the rules of evidence. It may be that the PX can be sued to civilly force it to comply with the law, or be ordered to cease surveillance activities in the absence of posting notices, yet their absence will not cause the evidence obtained from them from being used in a criminal prosecution. If you are facing criminal charges, hire competent defense counsel and have him or her research this issue. I doubt you will find an iron clad guaranty that the evidence will not be allowed by the judge. You will need to weigh the risks of trial against other potential options, such as an offered diversion program or plea agreement to lesser charges or penalties.
Thats a brilliant argument. But the answer depends on the nature of the reg, and whether the judge is military or a US magistrate. Contact a former JAG officer for accurate legal advice.