'American Idol' Castoff River St. James -- I'm Out on the Streets

Defying all laws of physics ... the "American Idol" judges actually had something useful to say ... because one "Idol" castoff is turning their advice into COLD, HARD CASH.

River St. James -- who was cut before the top 24 -- was told by the trio of judges to get more experience. Heeding their words, he decided to take his musical talents to the streets of Santa Monica, CA.

But River wanted to make sure to go the legal route -- and obtained an L.A. performer's permit. We're told he now plays 3-4 times a week on the 3rd Street Promenade, where he racks up tons of free practice ... and even earns a decent living.