It's not that they're walking away from it, as much as changing revenue streams. They are expanding their MinitClinic operations in-store, and reportedly will be doing smoking-cessation training, etc. Because they get a nice fee for the clinics, and bonuses from the Feds for each person they wean off tobacco.
Or: the short version: NOT selling tobacco, they'll make significantly more than selling it.
Or the ultra-short version: if the question is "why", the answer is invariably "money".
And I'm quite sure the same holds for corporate scientists defending atrazine, glyphosphate, etc. . .