People like helping. I like helping, and I'm not under the impression the vet is pulling in barrels of cash, but there's a limit. We have three dogs, two of whom we didn't ask for. One inherited from my mom, on her death, another an ill-advised impulse of a family member--taken in to insure the safety of the grandkids. And our own doggie. We're not taking them in for their annual well-doggie visits any longer--just for when the rabies shots come due. It's gotten out-of-hand, price wise.
We don't buy thousand dollar phones and expensive restaurant meals, nor $5 cups of coffee. I don't go to the doctor myself; DH has to, but he's on medicare so that's okay. I'm on Obamacare, for which I'm instructed to be grateful--I don't go to the doctor.
So yes, when we take the dogs to the vet we don't buy heartworm meds. I'm not buying them online either--not when I can get sheep dip for nearly nothing a dose--not even pennies. To me, it honestly feels like a bit of a scam--not by the vet, but certainly by the manufacturers/distributors.