If I understand you correctly, I think you have it backwards -- as I read it, Bell Labs saw math as having "lasting value," while computer science was just a practical application. Morris didn't want to look foolish, so he persisted with math long after he realized that he really wanted to be a programmer; unfortunately, after he gave up on mathematics as a vocation, he didn't have the fortitude to keep in touch with those who advised him otherwise.
I'm fairly certain it's this, given the history of CS and programming. It started as a branch of mathematics. As an academic field, even in the 1980s/1990s and in some places today, it was/is taught as a branch of math. And comparatively, programming/operating computers was not high prestige and was seen more as grunt work.
Interestingly, in some ways the people in Bell Labs were right: the mathematics is what objectively /lasts/. Consider: how much mathematics from the 1950s is still in use and relevant, versus how much written code is still in use and relevant? Mathematics (and algorithms, etc.) have a permanency to them that simply written programs don't.
