It's more about the fact that a join can change the shape of the result set. Even if the columns being projected aren't surfaced, the DB still has to process the joins to make sure the result set has the correct shape. For example, a human might know that a join will always be one:one or one:zero-or-one, but the DB has no choice but to make sure. Perhaps using subqueries instead of joins would work, but that gets ugly too.

(Or maybe my knowledge is outdated and the optimizers have gotten way better than they were 3-4 years ago.)