> Bug or feature ?

Feature.

> The group id is always
> propagated, even if the user which create the file isn't member of this group.

This is the BSD way to do it. The setgid bit thing you are describing
is used to map to the SYSV way of creating new files -- in SYSV they
files always take on the gid of the user.

You can look in the NetBSD mailinglist archives for previous discussions
on this.

------------------------------------------------------------------------------