The business to first class "accommodation charge" varies between different source/destination pairs. Between Boston and New York, I believe it is $61.
There are several answers to your question:
1. Business travel pays for it.
2. I'm using S+ upgrade vouchers.
3. For some heavily traveled days, the business class can be a mob scene.
4. Access to Club Acela, though S+ members already have access to it.
5. 750 vs 500 EQM points, plus elite bonus points. Faster to qualify for next year.
6. Last year's buy-2-get-1-free promotion had it such that one can earn a free first class travel, if all qualifying segments are in first class.
Did I miss anything?