Eduardo, what would you propose then?  That people have to commit on both the release and HEAD themselves?  
This doesn't work as well as expected as there are usually conflicts and they don't get resolved very well.
I am not sure this is really accurate; there have been 
some problems with merge conflicts, but the vast majority 
of the cases get handled properly and we have not lost 
any bug fixes on the release branches.
If your proposal is that people are responsible for merging when they commit bug fixes, I think the short answer is that 
it's not administratively tenable to do it that way unless
someone steps forward to take responsiblity for auditing commits to make sure that bug fixes do get rolled forward to 
HEAD.
The way we do it now may not be ideal but I think it's
the best solution with the resources we have now.