Why the logic of pull-out for join is integrated within the transformation of JoinExecTransformer

[jiangjiangtian]

Recently I read the code about the process of pull-out and I notice that the the logic of pull-out for join is integrated within the transformation of JoinExecTransformer, and I don't know if there is any problem to put the the logic of pull-out for join into the rule PullOutPreProject, just like other operators?
Does anyone know the reason? Thanks for answering!