Thanks for the thoughts - got the answer from Anthony of 'Jazzace' -
http://www.members.shaw.ca/jazzace/ace/hoop/
The short answer is that in the 'mental block' cases I described (put back from a rebound or shoot in from defensive end ball for example), even though they're deliberate shots at your own basket, they're considered 'accidental' and the points stand (as they always used to - this is the 'normal' case).
It's only when a team is clearly intentionally scoring in their own basket that the rule and violation/possession applies. I'll now quote Anthony for the reason the rule was introduced:
"The story of why the rule was created was told to me in a clinic by Fred Horgan, FIBA Technical Commission member from Canada. Apparently in an international competition, a certain team needed to win by 20 points to get a particular seeding. With a minute left, they were up by less than 10. They decided to score into their own basket until the score was tied, thus giving them five more minutes to generate that 20-point margin. A rule was needed to prevent this from happening."
Thanks again all - and particularly Anthony. His site (URL above) is great if you're not already familiar with it.