In my case, I needed an ITIN as I work for the Canadian branch of US based company who can send me to clients either in Canada or the US. I need to pay US taxes for the days I'm in the US at a US client.
But I don't believe the part about not needing the number is correct. The IRS requires those taxes to be filed. Failing to get the number, and filing, is basically, failing to file your taxes. The lack of number just makes it harder for them to track you down, but technically, you'd still be liable for filing in pretty much any case that's determined to be income in the US's eyes. However, I'll admit, as I haven't done a sale in the US, I don't know for certain in this case.