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Humans instinctively clench their fists when tense. If people are not covering the brakes it means their first reaction in an emergency is to grasp the bars, they then need to release and get the fingers to the brakes before they can stop. In an emergency it is better to be able to just pull the brakes immediately.
MTB training is also to cover the brakes (with one finger) to encourage a relaxed but secure grip of the bars. Too tense a grip impairs control and handing.
That is a really stupid thing to do. It takes a fraction of a second to move your hand to your brake and use it. Not having a full grip on the handlebars surely impairs control and handling.