They break this rule once. Old Biff goes back to 1955 and gives himself the Almanac but instead of returning to a future where he is rich he returns to the standard 2015 as if the changes never occurred.
That's not them breaking a rule. It's the same rule they worked with ever since the first movie.
The movie makers call it *The Ripple Effect*, and it's generally used when someone has travelled back in time and alters something. Think of the ripple effect as someone jogging behind your car as you are doing 70mph. You are driving so you are fine, but if you stop, the ripple effect will eventually catch up. It's shown in the first movie with the picture of Marty and his siblings fading.
In PT2, they are in a race to BEAT the ripple effect in 1985-C by finding out when the timeline changed and Biff became corrupt and powerful. The reason for this is for an important one. in THIS 1985, Doc is in the insane asylum, and NEVER built the Delorean. If the ripple effect catches up with them before they discover the date Old Biff went to, The delorean will fade from existence and they will disappear too.
I think I follow. So there's no predestination paradoxes here, entire timelines take place independently of eachother.
It's set up more like there is only one reality. But it can be changed. It slightly resists that change, hence the ripple effect(Technically, as soon as Marty was run over by his Granddad's car, he should have disappeared as, almost instantly, his parent's will now never meet). The ripple effect allows us to have a bit of drama in a movie.
The exact opposite is true in Looper. Cause and effect is instant. See the instance where young Joe shoots himself. The effect happens right away.