The difference is probably that some have a good accountant, and others don’t. If you make more than $600 and the US company doesn’t file a 1099 at the end of the year with your ITIN number, they are liable for the income tax on your sales, which I have been told is usually at a rate of more than 20%. The only other option is for the company to withhold that money from your payments, but it is more work for them and their accountant.
Disclaimer – I am not an accountant and you should get professional advice on this subject.
Instead of going in person to an embassy, you can call the IRS in the USA and get an ITIN over the phone, according to this discussion about iBooks:
https://discussions.apple.com/thread/3105682?start=15&tstart=0
I phoned this number (Apply for EIN by phone, international applicants): 1 (267) 941 1099
The 1 is American country code, 267 is the area code.
It literally took only 15 mins on the phone, including hold time, before I got my EIN number! It was very easy!