[xiii1408]

There's a little bit of truth to this--historically, lower doses of hydrocodone and oxycodone mixed with acetaminophen were a lower drug schedule in the US, allowing for less stringent requirements on security during production and transportation. Presumably, this was because the DEA thought there was less potential for abuse in the combinations drugs.

However, there's a more important medical reason: acetaminophen accentuates the analgesic affects of opioids, although the mechanism for this is not clearly understood. This makes acetaminophen-enhanced opioids more effective drugs.

Opioid abuse has become so rampant, though, that we're moving away from prescribing acetaminophen-enhanced opioids like Vicodin and Percocet to try to stop them entering the recreational drug market. As opioid use has increased, so has the number of liver toxicity deaths due to the acetamiophen in some prescription opioid drugs.