I liked Gaffer’s initial post, but you make an excellent point. To add to the dog-lover-vet-spouse-landlord.
Where is the consideration for the other tenants? Perhaps some of them moved there thinking “dog-free” means allergy-free, or even fear-free (yes, many people are fearful of dogs, I can only imagine a Mastiff). This is like advertising smoke-free hotels and apartments - only to find suddenly someone starts smoking in their rooms or down the hall. Worse when the owner makes the exception. If he wants to change his mind, he needs to warn the other tenants and give them time to opt out of their contracts or finish them.
The needs of the many outweighs the needs of the 1.
Maybe here it depends on the tenants - would they be OK with it?
I think what you lay out is exactly why some places say they’re dog-free, except for service or seeing-eye dogs. Apparently that exception doesn’t apply at this apt complex. I’d probably have to give the owner the benefit of the doubt on this one. Sometimes if you make one exception, a dozen others follow.