So, these foreign partners had US tax withheld and income reported.  They have US ECI and would be required to file a US tax return.  Without an ITIN, they can't claim a credit for the withholding (which supposedly goes into a pool) and would have a balance due on the US return.  At the same time, they will presumably have to report that same US-source income on their home country tax return.  Without documentary proof of their US tax payment, they would then be subject to double tax in their home country.  All because they don't bother to apply for an ITIN?

Something doesn't quite add up...