The question "can" the slab be rejected is strictly a contractural question. If the contract indicates a certain amount of air entrainment and gives authority to someone to direct that defective concrete be removed and replaced, and the concrete fails to meet the specifications, then the person with that authority "can" certainly reject the concrete. If the person indicated in the contract with acceptance/rejection authority is the "testor", then the testor can direct this. That would be unusual though, unless the testor is hired by the person with contractural authority and that person has delegated the authority to approve or reject the material to the testor. Normally it's either a design or agency engineer or architect, or perhaps a construction managament consulting engineer.
"Should" the slab be rejected is another matter that probably depends on the freeze/thaw conditions in the locale, and the general level of the contractor's cooperativeness in complying with contract requirements and providing quality work. If freeze/thaw is not an issue, the benefits of air entrainment are economy and ease of placement for the concrete supplier and contractor - not the durability of the concrete. If that is the case, there is really no reason to reject the slab.