Short answer: Because that's Apple's update policy.

Longer answer: Maintaining and supporting several major iOS versions is a lot of work, even if the older versions would just get security fixes. It's not just about the source code, any update also needs testing, dealing with apps which may break, update of various support documents etc. So you get both new functionality and security fixes by staying with the most recent version. In your case, running iOS 8 on an 4S actually works quite fine. 

Why do I have to update to the most recent iOS version?
I don't really get the absorption law, specifically in this case:

$$ (\lnot p \lor q) \land (\lnot r \lor q) \equiv  (\lnot p \land \lnot r) \lor (\lnot p \land q) \lor (q\land \lnot r) \lor (q \land q) $$

The absorption law is used here: $ (\lnot p \land q) \lor (q\land \lnot r) $

I don't really understand the use of absorption law in this case, and if someone could explain it to me on a similar example or something, that would be highly appreciated.
