In my opinion the problem is not that it is much more difficult to create a fastbreak out of a defensiv rebound.
My problem ist that when a grab the rebound I usually pass the ball quickly to my PG Iverson (not the slowest player I think

) and start sprinting down floor. But usually the cpu PF und C can catch me up and when I try a layup they block me. That is completly unrealistic and very annoying.
The only chance for a fastbreak is to intercept a pass, block a 3 pointer or simply steal the ball from the oppent PG. In this cases the cpu players paradoxically can't catch me up.
My conclusion is that EA tries to fix the problem (to many fastbreaks in Live 2004) by including a cpu assistance that makes it impossible to run fastbreaks out of the rebound. This kind of cpu assistance seems to be disabled when you earn you a fastbreak chance by good defense.