The whole point of the precautions (other than the remote connection of the final grounding connection) is to minimise the amount of surge that occurs when the 'dead' vehicles charging system suddenly sees a flat battery, is running at a fast idle following a cold start so is capable of a high output, and consequently spikes the ECU.
Leaving the vehicles running and connected as long as practicable achieves two things. Firstly, the 'dead' vehicles battery will have gained charge significantly. Secondly, the 'dead' vehicle will hopefully have warmed up enough for it to have dropped to idle. In both cases, surge is likely to be reduced.
It is worth remembering that modern cars have very high output alternators, and many of them use some form of Ford's 'Smart-charge' system, where the charging voltage can be higher than older systems.
I would be less likely to expect this to happen had I not been aware of two vehicles that I know it happened to.
The first was a relative's VW T4 that had not been started for a couple of weeks. The owner jumped it, and quickly pulled the leads off. It immediately stopped, and wouldn't restart. The garage diagnosed ECU failure, and had to fit a re-manufactured one.
The second was a neighbour's Pug diesel. The owner ran it out of fuel, and whilst attempting to bleed it (they are notoriously tricky) ran the battery down. He jumped it, and it eventually started, but died when he pulled the leads, and wouldn't restart. I couldn't convince him that the fault was unrelated to the initial lack of fuel, but after a week, he sent the ECU away for testing. It was dead; a re-manufactured unit had it running again.
It doesn't matter. If there is any hydrogen, and you can increase the distance to a source of ignition, why on earth wouldn't you do so?
They would vent exactly the same amount of gas as previous batteries. What do you think would happen if they didn't?
It's not a law, it's common sense based on the misfortunes of others. Read the recent post from steve robinson.
Chris