Not only gravity, but drag too.

What I always wondered about these kind of rocks: how come they can preserve anything useful (like that germ fossil) apart from some traits of their original chemical composition? If there's a big enough force to throw the rock off the planet, how come the heat probably close to the force not melt the crap out of it?

The objects are probably a 'reasonable' distance from the strike point of whatever impact caused them to be ejected from the surface which would help.

Secondly due to the speeds involved what is likely to happen is the outside is heated quickly but the object then moves away from the source of the heat at speed protecting whatever is inside the object. Think Crème brûlée in rock form

It's a bit of that, and a bit of an ablation issue. As the surface of the rock gets superheated, it will vaporized and get blown away. The vaporized rock creates a thermal barrier between the solid rock and heat source, drastically reducing the amount of heat the rock receives.

Not only gravity, but drag too.

What I always wondered about these kind of rocks: how come they can preserve anything useful (like that germ fossil) apart from some traits of their original chemical composition? If there's a big enough force to throw the rock off the planet, how come the heat probably close to the force not melt the crap out of it?

The objects are probably a 'reasonable' distance from the strike point of whatever impact caused them to be ejected from the surface which would help.

Secondly due to the speeds involved what is likely to happen is the outside is heated quickly but the object then moves away from the source of the heat at speed protecting whatever is inside the object. Think Crème brûlée in rock form

Yes, but the object has to speed up first, and for such a speed, wouldn't it has be close to the collision (how I hate English tenses), where things get vaporized?

Maybe it's due to some kind of scourge/lash effect when the rock is torn off from a bigger piece that acted as a heat shield.

+what Capn said

Time to take the English pills...