With the exception of a few of the training packages I believe most are discounted because they contain a certain amount of "background skills" i.e those that probably wouldn't have been purchased had the full amount of DP been required to be paid for them.
I believe that the TP aid in producing well-rounded starting characters with in-build backgrounds (or at least give a guideline for the characters history). While it is possible to abuse the system by stacking up ranks in specific skills I don't really think that in the long run this abuse matters to much for game play, those who do so generally hit the diminishing returns wall just a little faster.
TP's like the weapon master are the ones that needed to be fixed points-wise, because they simply over-emphasise a favoured skill that would have been purchased anyway, exchanging a few of the weapon skill ranks given in this TP for related lore skills might of made them a little less unattractive and a little more logical.
The material benifits to be gained (equipment mainly) is quite disproportiate between the various TP, and it is this, if anything that needs to be addressed with regard to discounting the TP.
In some ways perhaps the starting Training Packages should be paid for with Talent Points (or Background options which ever you use).