That's because Jamerson played upright too. Those open notes are a welcome option whenever you can use them for position shifts and when playing a 4 string you need to play more vertically up and down the fret board compared to a 5 string or more bass. Rahock beat me to it. The way Jamerson played out of key open notes made them almost sound diatonic. I've had the standing in the shadows of Motown book for a long time and even though it doesn't seem like it should work, when you can nail a line like "How long has that evening train been gone", it makes perfect sense.