What is the reason? It seems to be to train concentration while avoiding one pointedness? Might the lack of one pointedness be counter productive at some point?
The goal of the Mahasi method is 'vipassana', namely, seeing the arising & passing of phenomena. Therefore, beginners watch the rising & falling of the breath at the abdomen.
Often unskilled meditators force or suppress the mind when trying to practise Anapanasati (awareness of breathing), which results in a 'wrong one-pointedness', which blinds the mind & makes the mind sleepy & foggy.
Genuine or right one-pointedness occurs in jhana, which only meditators skilled in letting-go can genuinely reach.
Jhana cannot be reached by suppression or force, even though many Buddhists claim it can.
Such Buddhists are over-estimating their meditation because a suppressed/forced mind can generate calm & rapture but such momentary rapture is not the true rapture of true jhana.
Therefore, the Mahasi method aims for beginners to keep the mind 'open & awake' rather than 'suppressed, sleepy & foggy', which more closely represents the path taught by the Buddha.
In conclusion, it is forced/wrong 'one-pointedness' that is counter productive.