This behavior actually isn't well documented, since the semantic is
unclear in that documentation (or the Ruby Manual) what will happen in
the case that the grouping operator is used in this manner.

In a nutshell, newline delimits statements except when it doesn't
(trail with a binary operator for instance)... not a really good
semantic but I'll add that to the list of gotchas I need to deal with
in Ruby.

The 1.4 English Ruby Manual says:
Each expression are delimited by semicolons(;) or newlines.

The current Japanese Ruby Manual essentially says the same thing (my
Japanese isn't as sharp as it used to be):
式と式の間はセミコロン(;)または改行で区切ります
(shiki to shiki no aida wa semikoron(;)  mata wa kaigyou de kugirimasu)

-Almann