You had defined ‘amoralism’ as “mathematical non-cognitivism,” which means that mathematical sentences do not express ‘propositions’. However, almost all mathematicians express their sentences as ‘propositions’. Therefore, we can logically label almost all mathematicians as ‘moralists’ in the sense of “mathematical cognitivism.”
Almost all ‘amoralists’ (as in ‘ethical non-cognitivists’) who practice mathematics express mathematical sentences as ‘propositions’. However, they leave out possibility of this for ‘ethical’ sentences. Why do they avoid setting truth values to ‘ethical’ sentences when they do not avoid setting truth values to mathematical sentences?
The answer is: The ‘ethical non-cognitivists’ do not define ‘true’ and ‘false’ as ‘right’ and ‘wrong’, respectively. That’s it.
‘Ethical non-cognitivism’ does not imply ‘amoralism’ (in AC’s sense of the term), nor does it imply ‘moral subjectivism’ (in Juan’s sense of the term). ‘Ethical non-cognitivism’ only deals with the definition of ‘truth’. Therefore, your usage of ‘amoralism’ has nothing to do with Lilburne’s and zefreak’s conceptions of ‘morality’, and has everything to do with ‘ethical non-cognitivism’.