I somehow doubt it matters to the finances of the vet surgery whether a bitch is spayed at 6 months or a few months later after the first season...
I doubt they run such a hand to mouth existance that they need the fees for a spay op those few months earlier...
I think vets HAVE to get in with the "when are you going to neuter" question at the first appointment, as for the less responsible owners that is likely going to be the only time they see the animal (unless it is ill) and it is precisely these owners who risk having accidental litters which will only add to the number of animals in rescue.
It's a subject where there is always going to be disagreement.... the best you can do is research the pros and cons and then make up your own mind specific to you and your bitch...
We had Honey spayed at 7 months - before her first season.
We didn't just follow the advice of the vet, but did our own research too, and it was the right decision for us at the time.