@seafox69 -
Per the MA handbook in the link above, MA sourced income is defined as
All wages, salaries, tips, bonuses, fees and other compensation which relate to activities carried on in Massachusetts, regardless of where or
when the compensation is paid
if I live and work in WA as required by my MA based employer, there is no "activity carried on in MA"! so no MA tax. If I live and work in WA for my convenience as my employer otherwise requires me to be in MA ( I tele-commute!), I can see why MA taxes this as the activity should be occuring in MA! That is consistent with the Dept of Revenue response the OP above received.
Are you working in NH as required by your firm or are you working in NH for your convenience as your firm otherwise requires you to physcially be in MA each day? I can see that it would make a difference.
While I appreciate your reality and perspective, this issue of whether someone is working in a state for their convenience or as required by their employer has bearing on who the employee may owe tax to. It has become a complex subjust due to technology that enables tele--commuting
Just my two cents
