If I extend a word, do I get points for all the words, or just the straight line word?
You are correct (you get points just for SADE and EN, not for the other words still on the board that were on the board before your turn). Here's the rule from the official rules (emphasis added):


  The score for each turn is the sum of the letter values in each word(s) formed or modified on that turn


Perhaps an even easier way to resolve this doesn't even involve going back to the rules and finding the legal code that specifies it one way ot the other, but to consider what the ramifications of the other way of doing things would be.

Change your example a bit and the answer becomes clear:

If you add your E to 

S
ABOUT
DENTISTRY
 N
 D


to make SADE and EN again:

S
ABOUT
DENTISTRY
EN
 D


would you attempt to score ABOUT and DENTISTRY? I think not.