I don't see the point of criticizing them for lowering the entry costs. What if a bassist wants to play the game, but has no interest in playing the guitar portion of the game? If it was as you'd like it, then they'd have to buy the core game, and then buy the $30 DLC, just because they decided to play politics and not upset their original fanbase. They now have to pay $90 (slightly higher or lower depending on how much the disc is now) in order to play the bass portion of the game.
It's also the smart thing to do financially. Those who have not bought the game up to this point are much more likely to invest now that it features double the content. A far smaller number will be interested in buying the game, than paying for an expensive add on.
And frankly, insinuating that the original game was "unfinished" without bass could be seen as insulting. I have no idea if bass was in the works before release date, but as far as we knew, bass was never intended to be in Rocksmith, and was never advertised as such. It's like saying Rock Band is incomplete without DLC. This is not a patch, it's an add-on, meant to improve the game, not finish it.
I don't know, it all seems akin to children getting mad at the fortune of other children. "But he got more cake than I did!". Sheesh, do we really need to moan every time an opportunity opens for others that isn't available for us?