Ok, now I understand a little better what happened. I wouldn't assume it was your wording, though, LOL. I've been up the last couple nights due to pain, so am not reading sharp.
A vet usually is hesitant to share problems or issues with a given horse that he knows about from treating said horse. There are some unwritten guidelines that they need to be discrete and not share information...but...those unwritten rules give a dishonest seller an advantage.
Still, in general, it is the buyer who has to determine if the horse has a problem. If his prepurchase vet misses a problem, then he's going to have bought a 'lemon'. There is no assumption that the other vets involved in the horse over time, will or should or can, find the buyer, and warn him.
If people selling horses thought he routinely would kill a sale by sharing information they don't want public, they'd probably not use him themselves, and would bad-mouth the vet all over town, hoping other customers would shy away. So that might be the consequence of being 'honest'....going out of business.
But no matter what the consequences, there is usually a point where a vet will feel like he is getting pushed too far to 'keep quiet'. If s/he's sure the seller is misrepresenting the horse, he may quietly discuss it with another vet, and let that one make sure a seller knows what's really going on. There is often a little bit of an informal network among vets...they often hear a lot, and know the community, warts and all, pretty well. Sometimes they put aside risks to their livelihood and take a stand.
A trainer injures a horse, and the trainer's adoring student calls out the vet...the vet can see this could only have been intentional, or even worse, due to ignorance or loss of temper...and he's been out many times for similar problems....
A vet is called in to do a prepurchase exam, for a horse he knows has an untreatable issue that makes him unfit for buyer's purpose...and wouldn't turn up in a normal prepurchase....
A person with not enough money to board and train two show horses, calls the vet out to put a healthy horse with a mild tendon problem to sleep, because he can't be jumped at shows. The customer tells the vet to tell anyone who asks, that the horse developed EPM....
There is definitely a reason equine vets have a lot of stress!