Because we're not going to analyze the statement in the amount of depth needed to make that distinction before we crank up the transactional machinery. If it says SELECT, it gets a snapshot.
Ok, thanks. So to the original poster's point, perhaps the path with the least side effects / best Principle of Least Surprise (POLS) support is to start the transaction, and immediately call a "SELECT 1;" or perhaps better still, a 'SELECT timeofday();'