It does seem well established in the instructions for the NYS Form IT-203 and New York State Tax Law 631(b)(1)(B) that income derived from NY State sources such as a "business, trade, profession, or occupation" are taxable to nonresidents working outside of the state for their own convenience. These documents are less helpful regarding nonresidents that reside & work wholly without the State for the entire year (meaning 0 days per year within NYS), as you've explained. Rather, the rules set forth in 20 NYCRR 132.4(b) seem to indicate that income earned by a nonresident completely outside of NYS should not be counted towards their NYS AGI:
"The New York adjusted gross income of a nonresident individual rendering personal services as an employee includes the compensation for personal services entering into his Federal adjusted gross income, but only if, and to the extent that, his services were rendered within New York State. Compensation for personal services rendered by a nonresident individual wholly without New York State is not included in his New York adjusted gross income, regardless of the fact that payment may be made from a point within New York State or that the employer is a resident individual, partnership or corporation. Where the personal services are performed within and without New York State, the portion of the compensation attributable to the services performed within New York State must be determined in accordance with sections 132.16 through 132.18 of this Part."
This is contrary to what some others have posted where all NYS-sourced income should be taxed when the employee is working remotely for their own convenience. If anyone has more insight into this mater regarding the NYS Codes, Rules, & Regulations segment, it would be great to hear more.
Lastly, it's worth noting that Matter of Kumar (ALJ May 6, 2010) presents an issue where a NJ employee was charged taxes on income said to be related to a NY sales office. The ALJ ultimately determined that no tax was due, however, because the individual worked 0 days in NYS. This situation is arguably a bit different than the conditions being discussed, though, as the main office of the employer in this instance was located in Illinois and only the sales office was in NYS.