If every other payment provider accepts the vendor's terms and that the vendor already has KYC details on file with its US-authorised payment gateway, AMEX requiring signed scanned copies of its own payment terms and multiple forms of identity to be individually emailed to them for each and every transaction is the definition of 'extra hoops', and it seems quite reasonable to consider it difficult and obnoxious.
For a lot of transaction sizes it isn't really worth the extra effort, particularly as filling in forms like the W8 BEN E is the kind of work people tend to pay their accountant - quite possibly more than the value of the SaaS subscription - to do. (As someone who happily prepares his own tax returns once a year, the W8 BEN E is not a particularly straightforward form to fill in for comparatively little benefit)
For a lot of transaction sizes it isn't really worth the extra effort, particularly as filling in forms like the W8 BEN E is the kind of work people tend to pay their accountant - quite possibly more than the value of the SaaS subscription - to do. (As someone who happily prepares his own tax returns once a year, the W8 BEN E is not a particularly straightforward form to fill in for comparatively little benefit)