LemmyM is correct in his assumption.
For example, if your race researches Quantum Electro Dynamics they may not be able to research Flux Capacitance.
or (a rather more extreme example)
if they researched "advanced religion" they may not be able to research "science"
As for current restrictions, I think you can only restrict techs based on racial traits you have selected... which IMO sucks
My suggestion was that it would be good to ADD "anti-requirements" to the game
As for the assigning tech to all races individually... well it would be a lot of hard work but possible.
However, it would probably be easier to do it the way I suggested (or at least a lot faster)
Matryx