I think most states do it that way. Sorry.
Most states compute the tax only to the income attributable to their state for non-residents. Part year residents you are correct. They are different.
“Most states compute the tax only to the income attributable to their state for non-residents.”
Actually, most of the states I know of compute non-resident tax using the federal AGI as the basis for their tax, and then adjust it using the money earned in state to calculate a percentage. Arizona does it that way. Colorado. Oklahoma. That is 3 for 3 of states I just checked. Apparently NY does it as well.