Great question! The phrase “legally acceptable in the USA” is relative to the instructions or guidelines of the document or agency.
For instance, some documents/agencies may require “UETA/ESIGN compliant”, however, for the W-9 the signature is required to be from a system that “authenticates and verifies the submission“, which a WordPress user account accomplishes.
If you have more questions about the signature add-on plugin, open a support ticket on FormidablePro.com as they know their plugin, and its legality, much better than we do.