Now I may be wrong on this since I've never been one to read books but there are a number of reasons to assign low + and Hi - values.... First and most simple would be your betting spread. When low card (or +) is high, it is your signal to increase your bets in a positive way. The most important reason to assign a + for low card is, depending on the system you are planning on using, you may need to calculate your running count by the how many decks remain to come up with your True Count. In most systems, your True Count is what you will be using to increase and decrease your bets. Also, in other unbalanced systems you will start with a running count of a positive number above zero. If you are using OPP, Ace/5 or Speed Count this may seem Bass Akwards but trust me, it will become necessary if your game advances to more productive counting systems.
Other reasons would be calculations which I need a real crash course in ;).
for the player, because there are more high cards remaining in
the deck(s). When high cards come out it's a negative for the
player, because there is less chance of making a BJ and winning
150% of your bet.
On the surface it could appear backwards, but it's not. That's
why people study the game. Quite a few intuitive plays/ideas are
actually mathematically incorrect.
You should be more interested in the value of the cards in the undealt portion of the deck (shoe), as you are far more likely to receive these cards in the next hand than the ones in the discard tray. Although I first read of this in a book, it is confirmed by personal experience.