The reason you need to to this is that machine thread is oiled to make it go through the machines easier. This is why often clothes will feel very soft when you purchase them, but will feel less soft after they have been laundered a few times.
I prefer the boiling method to prep cloth diapers, even though it takes more work on my part. It takes less time and energy, and you get to see why it is necessary to do this.
I have a pot that I use for crafts. It is no longer used for anything we will be eating. I use it to dye yarn or clothes, or prep diapers. I fill it about 3/4 full of water and set it to boil. Give it a squirt of soap.
Compare the two pictures.