said by bt:PlayOn does show the in-video ads from Hulu when the stream is forwarded just like you would see on the PC.
In general, it's not that weird. "Free" isn't exactly what those shows are - they're ad supported. Delivering those shows via devices that aren't showing the on-page advertisements would hurt Hulu's (likely razor thin) profit margins.
This specific case is definitely weird though, because it's just another full browser - ads and all.