Do you know which manufacturers say this?
In addition to those already posted:
Kask: "Never wear anything under your helmet except for a winter cap accessory specifically designed for your helmet and supplied by your helmet manufacturer." (way to corner that winter cap market, Kask!)
Trek/Bontrager: "Anything between your head and the helmet can reduce the protection. A cap, scarf, or even some high-volume hairstyles might allow the helmet to move during an impact. Barrettes, headphones, or anything else under your helmet can injure you if the helmet is impacted."
Giant: "YOU SHOULD NEVER ... Wear anything under your helmet"
Generally, helmet manufacturers clearly say that even the pathetic warranties that they do offer (if your loved ones can prove that a defective helmet contributed to your demise and have proof of purchase, they might get the price of the helmet back!) are voided in the case of product misuse - presumably such as having "high-volume hairstyles" under it. Do we know of anyone ever successfully claiming for a crashed defective helmet?
Poc helmets don't seem to have a manual online. I guess Poc helmets don't care about informing customers and don't want them to know the restrictions on Poc helmet use until
after they've got the money!