It's likely because it has a cord and is considered an extension cord. I have the one below and had no issues. I also leave the original packaging wherever I have it plugged in so the stateroom attendant knows it's not a surge protector.

https://smile.amazon.com/gp/product/B07X45J85H

EDIT: The one at the link is no longer available apparently, but one like it without the cord would likely be fine.