Seamless browsing context in object elements

Hi everybody.
The title says all, so this time, short request and therefore short message.
The specification says that object elements' associated resource can be
treated as a nested browsing context. But it can't be specified with
<iframe> new S-attributes (@srcdoc, @sandbox and @seamless). I can
understand the reason for @sandbox, as I guess that sandbox limitations
could be incompatible with plugins. I can understand why not @sandbox as
well (that's good for a very specific context I think). But what about
@seamless? Is there a reason why it does not apply to <object> browsing
contexts?
Thank you very much for your patience.
Andrea Rendine