Why, because Bennett couldn't be convicted of it? Bennett took decades off Shumaker's life. AFAICT it's for practical reasons that he got a lesser charge, not for philosophical reasons.
I think that particular exception to double jeopardy is unique to New York. It’s not my area of expertise so I could be wrong but I think most of the country still uses some version of the “year and a day” rule.
[0] https://www.wionews.com/world/patient-in-ground-breaking-hea...