Working in the veterinary profession I'd like to add some things...
(1) Office managers should not have the power to over-rule the veterinarian, but some vets are basically bullied by practice managers/owners - it is not as common in a good practice (From my own experience I will only work or take my pets to AAHA Certified Vets [~15% of the nations clinics]. AAHA clinics go through a rigorous credentialing process and every 3 year inspections)
(2) Legally they can't force you to buy from them, unfortunately it sounds like the management has decided not to adhere to ethics in this case and hopes no one questions them too much- a vet can loose their license for refusing treatment unless the reasons are explicitly unethical and in these cases it has to be documented extensively for legal reasons - de-barking dogs, euthanizing a healthy pet for convenience, removing teeth / claws for biting or furniture scratching - though declawing cats is still legal in the US some refuse to do it due to the immense pain the cats go through and how unnecessary it is
(3)While I agree that a script could easily have been written since ceftazidime isn't a controlled drug (don't get me started on all the FDA hoops for that!)- it is also a sensitive medication that needs reconstituted and stored properly in order to have any effect. I'm sure you are smart enough to follow directions since you are comfortable giving injections, but soooo many people just can't. Meds that are pre-preped at the needed dose have less margin of error.
(4) Yes the reason most vets prefer you get meds from them is to make money - it is a business after all and the pharmacy is a big chunk, just like surgery - clinics loose a lot of money to both human and online pharmacies - but also because there ARE online pharmacies and warehouses' that don't store meds correctly and then you risk your pet getting the meds this way.
I have direct experience with a client buying insulin from a popular online pet pharmacy, we couldn't figure out why the pet's blood sugar was so hard to balance. She was swinging between so low as to be nearly unconscious to so high to be having seizures...when hospitalized on an insulin drip she was stable. As soon as she went home, her glucose would go crazy again. After weeks of being in and out of the hospital and the owner being at her wits end, we finally found out that her insulin was not being stored or shipped refrigerated.
On a final note, the vets themselves don't get kickbacks for things like prescription food and medications. At most the clinic gets free samples and catered lunches from the drug reps. There are safe online pharmacies out there, if your vet works with them. I can PM you more info if its wanted
Most of us care about your pets' health not making money, compared to human medicine they make much less money on average. There are some out there that don't, but when you find those ones, just change vets. Specialists will tend to get paid more, as since they have done the schooling their services tend to cost more. Getting an MRI for you dog is going to cost more then getting xrays at a regular practice.