Hi, Sorry, been away a while. Greg correctly explains the FIA's "logic", but I believe the change is more driven to close up the field - as FRIC has been around for quite a while without objection. The real issue is that some teams really understood what could be done, and took advantage of it, others were not so capable.
If well understood and optimized, the system can be set up to "couple" the front/rear as a tuneable variable - ie you can make it act as front in, rear goes (or tries to go) in, or front in, rear goes ( or tries to go) out. The classic passenger car adoption was in/out, which promotes a flat attitude over wheel inputs such as a sleeping policeman - but accentuates brake dive. The opposite, in/in coupling can be used to reduce pitching due to body applied loads such as aero or braking and most folks tend to think of this latter method being used in F1 - but more can be done...
Why the coupling is important is that you can set it up to manage the car attitude as the aero loads change. For example, if the rear aero force grows at a greater rate with speed than the front (or rear is more softly sprung), using the rear drop in height to force a similar drop in front height will tend to keep the car level (in/in coupling). However, alternatively you could accentuate the pitch change due to aero - making the rear drop cause a rise at the front (in/out couping), reducing wing attack angle and hence down/force and drag.
This is where the system gets complicated as each track will benefit from different strategies - a point and shoot track might be best with in/out, to give low drag on the straights and big downforce during braking and in the turns - whereas a fast sweeping track would want in/in, giving a more stable platform. It's quite hard to change the set-up once at the track (physical components have to be changed), so the modelling has to be very accurate. I have seen some diagrams that hint at switchable coupling modes, but I don't see how that could be legal.
Why I suspect the FIA is banning it now is because Mercedes have it working too well and the racing is suffering, and/or the boys in red are complaining. ( Ferrari used FRIC too but they can't seem to make it work so well - so the ban to them is a good thing).
Andrew