No they aren't :) They alias just like for/foreach.
I know that they alias the same way. But for some reason, I found that nesting foreach statements with the default $_ variable often seems to lead to confusions between the internal and the external values of $_, while I do not have this problem with nested map statements. But it is also possible that I simply don't do it right with the nesting of foreach statement. I don't do that too often anyway, I usually prefer to use explicit variables when nesting such statements in production code.