If you don't want them regulated into the expensive category, then I would think you would be fully behind industry self-policing. With more and more people vaping, statistically there is a greater risk of somebody having side effects/adverse reactions. If you're (in the general sense, not you specifically) going to condone and even encourage using diacetyl, a known respiratory toxin, you might as well send the FDA an engraved invitation to come in and regulate.
For example, more and more smokers are changing over to vaping with all levels of lung disease already. Goes to the doctor's office. "Dr, I quit cigarettes using ecigs but I'm still short of breath." Doctor talks a little bit and finds out the patient is vaping a buttery cinnamon caramel roll. Despite what some may think, physicians are not stupid, they do talk to each other, and their regular meetings of the AMA are about more than just having a party. Doctor knows that inhalable butter means diacetyl. He mentions it to a colleague, compare notes, and it snowballs all the way up to an incident report sitting in the "woohoo we got 'em now" file a FDA.
There is no need for diacetyl to be in efluid and replacing it will not drive up the cost. If there some sort of industry organization, there would be discounts available on testing, and if a company is operating on such a thin margin that testing is cost prohibitive, well, you have to wonder what other cost-saving shortcuts they may take. And, if you think about it more long-term, what's going to happen when diacetyl butter cinnamon caramel vapors go back and sue the ejuice vendors the same way tobacco smokers did? Which is going to be more cost effective to the vendor?