You're not forced to brake if you can see that 2 lanes go down to 1 and you therefore anticipate that the cars in the other lane are going to have to merge into your lane and you leave a gap to allow them to do so.
It is a mistake to think that one of the two input lanes is more entitled to the output lane than the other. 2 lanes go down to 1, neither is more privileged than the other, cars should merge in turn.
Your comment makes less sense when the merging lane has no one else it as well. That means the car in it that is merging over has plenty of opportunities to merge safely without causing traffic congestion however they don't.