Watch F1 sometime, How many rear end collisions do you see throughout a race? Not many, The reason drivers take different lines to pass their opponents, On the flip-side of that how many rear end collisions do you see in the BTCC? A lot because they are basically attached bumper to bumper.
Not everybody understands racecraft, If I know a long straight is coming up on the exit of the next corner I'll be braking early to take an early apex and be accelerating down that straight sooner than the guy following me.
Not sure what you're saying? You seem to be disagreeing and agreeing with me in the same sentence? Sure, F1 cars don't always follow super close, because the cars are so fragile and the aerodynamics are so important so they don't want to rip the nose cone off, but the BTCC cars are much closer to what we're racing in GTS. They have bodywork and can bump each other without much damage occurring. Perhaps if Sport mode included full blown damage then people would be more careful, but it could go the other way and you could get punted into the wall and have a severely damaged car whilst the guy who punted you had minimal bumper damage.
As you say though, not everybody understands racecraft, so whilst you might know to brake early to take an early apex, the person following you might not be expecting you to brake so early because most people don't. In such circumstances they're probably just as annoyed when you unexpectedly slam on the brakes and they get a penalty, as you are when they hit the back of you. If somebody is up your backside though, you're just as much to blame if you notice their proximity (as you appear to have done) and then still slam your brakes on early.
You just have to be aware of everyone around you and be expecting them to make a dick move. If you know the guy following will punt you at the next corner, jink to one side and immediately hit the brakes hard and early - they will probably go sailing past and off the track/into the wall and you'll be able to take a nice sedate turn and put some distance between you both.
However, the biggest thing you need to realise is that you will get hit from time to time, whether you like it or not. The same can be said in real life motorsports - how many times do you see F1 drivers getting tangled up with each other - and they're top paid professional race drivers at the top of their game, not a bunch of casual PS4 players.
I race with a friend who enjoys the game, but is useless at driving games. He often brakes far too late and often loses control and spins out. Usually he only takes himself out and drives over-cautiously online, but it wouldn't take much for him to accidentally rear end a pack of cars in front of him if he got a little over eager. Not every punt is intentional - sometimes people just out brake themselves massively or lose control.