I had to leave Zoni overnight when she was spayed. It threw me for a loop because I did not remember that occurring with Nemo's procedure (7 years ago so memory is a little fuzzy). When I asked the vet tech, she said it was a requirement for their AAHA accreditation. Didn't check into it further since it was moot at that point.
You need a new vet. I just CALLED (woman named Ruthie) AAHA
American Animal Hospital Association
12575 W. Bayaud Ave.
Lakewood, Colorado, USA 80228
Member Service Center:
She said (as I already KNEW but didn't want to load you without double checking) that they do NOT have any such requirements, period. They do not tell veterinarians how to manage their patients or run their business and no such regulations are in place that could even be misinterpreted.
IOW, your vet tech lied to gouge you for overnight fees, AND usually because they think owners are too stupid not to go play frisbee with a newly spayed/neutered dog. CALL THE VET and tell them what the vet tech told you and request a full refund for any overnight boarding. And threaten to file a complaint with AAHA if they don't. Perhaps your standing up will help stop them from telling lies and bullying the next client.