When can licenses be revoked? (was: License Proliferation)

[Ben Tilly]

On 9/6/05, Ben Tilly <btilly at gmail.com> wrote:
[...]
> I wouldn't like to be relying on public domain code, only to have the
> author of that code die and the inheritor decide that the public
> domain grant wasn't valid, and as current copyright holder s/he wants
> the copyright enforced.
[...]

Sanjoy Mahajan asked me an excellent question about this.  He asked
why a normal copyright license cannot be revoked by an heir.  If it is
a contract, that is fairly obvious.  But many popular licenses (eg
GPL, BSD) are not contracts.

I didn't know, but it was my impression that it couldn't happen. 
However when I googled for the topic, I found nothing saying that it
couldn't happen, and http://www.ilaw.com.au/public/licencearticle.html
saying that it definitely could.

Is this really true?  And if so, does anyone have any thoughts on why
it isn't more often discussed?

Thanks,
Ben