I believe the point is to avoid overheating (any of) the milk, because that changes its flavour. For example, tea made with UHT milk tastes very different (and,

I believe the point is to avoid overheating (any of) the milk, because that changes its flavour. For example, tea made with UHT milk tastes very different (and, to me, much less nice) than tea made with pasteurised milk. When adding the tea to the milk, the temperature of the milk will never exceed the final temperature of the mix. When adding the milk to the tea, it will.