I did something about it on RTU - I've just forgotten what.  Since tar came from research which did have that kernel behavior, I'm guessing we modified tar when in read (input) mode  to not call chown unless it was run as root. 

To fix  the other issue with importing a tape that could be an issue, we (mil did the code it I remember correctly) also added a way to strip the leading slash from tar images on input so if some one wrote a tape with full paths you could import it path relative - which the research/BSD versions did not do - which until pax came along and later gnutar was not easy to do.

Clem