Reverse traceroute and spoofing of sources of packets?

I missed this meeting/preso when it happened (yes, 5+ meetings ago) <http://www.nanog.org/meetings/nanog45/abstracts.php?pt=MTE4OCZuYW5vZzQ1&nm=nanog45> I note the talks about using spoofed source packets to do some measurement, I didn't see anyone in the video say: "But spoofing is bad, but you want YOU to be able to do this? what? isn't that a little hypocritical?" Isn't it? and why would someone (an isp) disable BCP38 for this sort of activity? -Chris