"Normal Vets", or for that matter "Abnormal Vets" are only as good as their ability to apply their knowledge.
The question you should ask is why they believe it's a simple urinary tract infection as opposed to the beginning of kidney failure either separate or associated with an initial infection? There's certainly nothing wrong in waiting 48 hrs after starting Antibiotics . If it's a UT infection then the dog will perk up rapidly. Sounds like the Vet(s) are rushing things a little in this case.