Linda is a product of the world of "Our Ford". She had been embarrassed by even giving birth to John because "motherhood" was a dirty word in the Brave New World. During her time at the reservation, she still hung on to the values of that world by sleeping with other women's husbands and telling John how wonderful the Brave New World was. She never tried to adapt to its culture. Once Linda has a chance to return to the "civilized" world, she does not really want the embarrassment attached to having given birth to a child. She simply wants to escape into a world of soma and drug-induced happiness. Since she never saw or was taught the responsibilities of motherhood, she assumes John will be able to take care of himself. So, she abandons her child and allows herself to die of a soma overdose.
In BNW John is something like the bearded lady ... a side show act. All the people who have never seen a "Savage" are interested in seeing him; this is not the case with Linda. For her, his mother (... did I really say that :)), he is just John, part of her life, part of the "normal" life on the reservation. I think this fascination with the abnormal is very common and nothing suprising for their world.