You said "typically (not always)" and I say pretty close to never.
I don't try to justify the tiering of the co-pay
You said the co-pay was tiered "because such procedures are typically (not always) preventable" (emphasis mine).
(i'm note 100% sure that by 'tiered' you mean that you pay 50% for this instead of the flat or 0 co-pay most dental coverage has for checkups and the like)
If I were trying to explain the rationale in this case, I'd theorize it's because this is typically something that happens once in a lifetime (if at all). I'd expect once one wisdom tooth gets cranky, they'd yank the rest at the same time. So, in this case, the thought would be that people partially pay for it when it happens rather than making everyone in the insurance pool pay for that risk every month, especially people with no wisdom teeth.
But I am not an actuary or insurer, so I don't know for sure.
Dental insurance is pretty cheap -- there are a limited number of high-dollar procedures that happen infrequently. Plus, major costs like anesthesia are usually part of medical coverage. So while the insurance pool is more likely to pay for a wisdom extraction for many folks between 18 and 30, that's a one time event.
Medical insurance, on the other hand, needs to cover a broad array of overpriced procedures with nearly unlimited liability. I had a back surgery that cost my insurance company about $75k. The recent birth of my child cost nearly $25k!