Yes, it is legal, if there is reason why the technician believes, based on her experience and/or training and the physical evidence (e.g. color; odor, or lack thereof) that that sample is not an authentic one (e.g. has been doctored or diluted; is some other substance; is not a "fresh" one and so was provided by someone else and brought in by the employee). They do not have to except what they believe to be an altered or inauthentic sample. Remember also, there is no right to employment ("employment at will"), so the employer can reject an applicant if it feels something is "off" or "wrong" about any part of the screening process.